Squashed from the following commits:
core/state: lazily init snapshot storage map
core/state: fix flawed meter on storage reads
core/state: make statedb/stateobjects reuse a hasher
core/blockchain, core/state: implement new trie prefetcher
core: make trie prefetcher deliver tries to statedb
core/state: refactor trie_prefetcher, export storage tries
blockchain: re-enable the next-block-prefetcher
state: remove panics in trie prefetcher
core/state/trie_prefetcher: address some review concerns
sq
* all: freezer style syncing
core, eth, les, light: clean up freezer relative APIs
core, eth, les, trie, ethdb, light: clean a bit
core, eth, les, light: add unit tests
core, light: rewrite setHead function
core, eth: fix downloader unit tests
core: add receipt chain insertion test
core: use constant instead of hardcoding table name
core: fix rollback
core: fix setHead
core/rawdb: remove canonical block first and then iterate side chain
core/rawdb, ethdb: add hasAncient interface
eth/downloader: calculate ancient limit via cht first
core, eth, ethdb: lots of fixes
* eth/downloader: print ancient disable log only for fast sync
With this commit, core/state's access to the underlying key/value database is
mediated through an interface. Database errors are tracked in StateDB and
returned by CommitTo or the new Error method.
Motivation for this change: We can remove the light client's duplicated copy of
core/state. The light client now supports node iteration, so tracing and storage
enumeration can work with the light client (not implemented in this commit).
