core: implement background trie prefetcher

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