feat: ADR 040: Implement in-memory DB backend (#9952)

## Description

Implements an in-memory backend for the DB interface introduced by https://github.com/cosmos/cosmos-sdk/pull/9573 and specified by [ADR-040](https://github.com/cosmos/cosmos-sdk/blob/eb7d939f86c6cd7b4218492364cdda3f649f06b5/docs/architecture/adr-040-storage-and-smt-state-commitments.md). This expands on the [btree](https://pkg.go.dev/github.com/google/btree)-based [`MemDB`](https://github.com/tendermint/tm-db/tree/master/memdb) from `tm-db` by using copy-on-write clones to implement versioning.

Resolves: https://github.com/vulcanize/cosmos-sdk/issues/2

Will move out of draft once https://github.com/cosmos/cosmos-sdk/pull/9573 is merged and rebased on.

### Author Checklist

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please add links to any relevant follow up issues.*

I have...

- [x] included the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [x] added `!` to the type prefix if API or client breaking change
- [x] targeted the correct branch (see [PR Targeting](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#pr-targeting))
- [x] provided a link to the relevant issue or specification
- [ ] followed the guidelines for [building modules](https://github.com/cosmos/cosmos-sdk/blob/master/docs/building-modules) n/a
- [x] included the necessary unit and integration [tests](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#testing)
- [x] added a changelog entry to `CHANGELOG.md`
- [x] included comments for [documenting Go code](https://blog.golang.org/godoc)
- [x] updated the relevant documentation or specification
- [x] reviewed "Files changed" and left comments if necessary
- [ ] confirmed all CI checks have passed

### Reviewers Checklist

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your handle next to the items reviewed if you only reviewed selected items.*

I have...

- [ ] confirmed the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] confirmed `!` in the type prefix if API or client breaking change
- [ ] confirmed all author checklist items have been addressed 
- [ ] reviewed state machine logic
- [ ] reviewed API design and naming
- [ ] reviewed documentation is accurate
- [ ] reviewed tests and test coverage
- [ ] manually tested (if applicable)

db/memdb/iterator.go

@@ -0,0 +1,139 @@
+package memdb
+
+import (
+	"bytes"
+	"context"
+
+	tmdb "github.com/cosmos/cosmos-sdk/db"
+	"github.com/google/btree"
+)
+
+const (
+	// Size of the channel buffer between traversal goroutine and iterator. Using an unbuffered
+	// channel causes two context switches per item sent, while buffering allows more work per
+	// context switch. Tuned with benchmarks.
+	chBufferSize = 64
+)
+
+// memDBIterator is a memDB iterator.
+type memDBIterator struct {
+	ch     <-chan *item
+	cancel context.CancelFunc
+	item   *item
+	start  []byte
+	end    []byte
+}
+
+var _ tmdb.Iterator = (*memDBIterator)(nil)
+
+// newMemDBIterator creates a new memDBIterator.
+// A visitor is passed to the btree which streams items to the iterator over a channel. Advancing
+// the iterator pulls items from the channel, returning execution to the visitor.
+// The reverse case needs some special handling, since we use [start, end) while btree uses (start, end]
+func newMemDBIterator(tx *dbTxn, start []byte, end []byte, reverse bool) *memDBIterator {
+	ctx, cancel := context.WithCancel(context.Background())
+	ch := make(chan *item, chBufferSize)
+	iter := &memDBIterator{
+		ch:     ch,
+		cancel: cancel,
+		start:  start,
+		end:    end,
+	}
+
+	go func() {
+		defer close(ch)
+		// Because we use [start, end) for reverse ranges, while btree uses (start, end], we need
+		// the following variables to handle some reverse iteration conditions ourselves.
+		var (
+			skipEqual     []byte
+			abortLessThan []byte
+		)
+		visitor := func(i btree.Item) bool {
+			item := i.(*item)
+			if skipEqual != nil && bytes.Equal(item.key, skipEqual) {
+				skipEqual = nil
+				return true
+			}
+			if abortLessThan != nil && bytes.Compare(item.key, abortLessThan) == -1 {
+				return false
+			}
+			select {
+			case <-ctx.Done():
+				return false
+			case ch <- item:
+				return true
+			}
+		}
+		switch {
+		case start == nil && end == nil && !reverse:
+			tx.btree.Ascend(visitor)
+		case start == nil && end == nil && reverse:
+			tx.btree.Descend(visitor)
+		case end == nil && !reverse:
+			// must handle this specially, since nil is considered less than anything else
+			tx.btree.AscendGreaterOrEqual(newKey(start), visitor)
+		case !reverse:
+			tx.btree.AscendRange(newKey(start), newKey(end), visitor)
+		case end == nil:
+			// abort after start, since we use [start, end) while btree uses (start, end]
+			abortLessThan = start
+			tx.btree.Descend(visitor)
+		default:
+			// skip end and abort after start, since we use [start, end) while btree uses (start, end]
+			skipEqual = end
+			abortLessThan = start
+			tx.btree.DescendLessOrEqual(newKey(end), visitor)
+		}
+	}()
+
+	return iter
+}
+
+// Close implements Iterator.
+func (i *memDBIterator) Close() error {
+	i.cancel()
+	for range i.ch { // drain channel
+	}
+	i.item = nil
+	return nil
+}
+
+// Domain implements Iterator.
+func (i *memDBIterator) Domain() ([]byte, []byte) {
+	return i.start, i.end
+}
+
+// Next implements Iterator.
+func (i *memDBIterator) Next() bool {
+	item, ok := <-i.ch
+	switch {
+	case ok:
+		i.item = item
+	default:
+		i.item = nil
+	}
+	return i.item != nil
+}
+
+// Error implements Iterator.
+func (i *memDBIterator) Error() error {
+	return nil
+}
+
+// Key implements Iterator.
+func (i *memDBIterator) Key() []byte {
+	i.assertIsValid()
+	return i.item.key
+}
+
+// Value implements Iterator.
+func (i *memDBIterator) Value() []byte {
+	i.assertIsValid()
+	return i.item.value
+}
+
+func (i *memDBIterator) assertIsValid() {
+	if i.item == nil {
+		panic("iterator is invalid")
+	}
+}