* Improve error handling in block iteration
* Introduce atomic DB operations
* Fix race condition
An invariant was violated: For every block hash in head_tracker, that
block is accessible from the store.
* Address compiler warning
* Prune abandoned fork choice forks
* New approach to pruning
* Wrap some block hashes in a newtype pattern
For increased type safety.
* Add Graphviz chain dump emitter for debugging
* Fix broken test case
* Make prunes_abandoned_forks use real DiskStore
* Mark finalized blocks in the GraphViz output
* Refine debug stringification of Slot and Epoch
Before this commit: print!("{:?}", Slot(123)) == "Slot(\n123\n)".
After this commit: print!("{:?", Slot(123)) == "Slot(123)".
* Simplify build_block()
* Rewrite test case using more composable test primitives
* Working rewritten test case
* Tighten fork prunning test checks
* Add another pruning test case
* Bugfix: Finalized blocks weren't always properly detected
* Pruning: Add pruning_does_not_touch_blocks_prior_to_finalization test case
* Tighten pruning tests: check if heads are tracked properly
* Add a failing test case for a buggy scenario
* Change name of function to a more accurate one
* Fix failing test case
* Test case: Were skipped slots' states pruned?
* Style fix: Simplify dereferencing
* Tighten pruning tests: check if abandoned states are deleted
* Towards atomicity of db ops
* Correct typo
* Prune also skipped slots' states
* New logic for handling skipped states
* Make skipped slots test pass
* Post conflict resolution fixes
* Formatting fixes
* Tests passing
* Block hashes in Graphviz node labels
* Removed unused changes
* Fix bug with states having < SlotsPerHistoricalRoot roots
* Consolidate State/BlockRootsIterator for pruning
* Address review feedback
* Fix a bug in pruning tests
* Detach prune_abandoned_forks() from its object
* Move migrate.rs from store to beacon_chain
* Move forks pruning onto a background thread
* Bugfix: Heads weren't pruned when prune set contained only the head
* Rename: freeze_to_state() -> process_finalization()
* Eliminate redundant function parameter
Co-authored-by: Michael Sproul <michael@sigmaprime.io>
