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

core/state/database.go

@@ -129,12 +129,20 @@ type cachingDB struct {
 
 // OpenTrie opens the main account trie at a specific root hash.
 func (db *cachingDB) OpenTrie(root common.Hash) (Trie, error) {
-	return trie.NewSecure(root, db.db)
+	tr, err := trie.NewSecure(root, db.db)
+	if err != nil {
+		return nil, err
+	}
+	return tr, nil
 }
 
 // OpenStorageTrie opens the storage trie of an account.
 func (db *cachingDB) OpenStorageTrie(addrHash, root common.Hash) (Trie, error) {
-	return trie.NewSecure(root, db.db)
+	tr, err := trie.NewSecure(root, db.db)
+	if err != nil {
+		return nil, err
+	}
+	return tr, nil
 }
 
 // CopyTrie returns an independent copy of the given trie.