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

crypto/crypto.go

@@ -60,10 +60,23 @@ type KeccakState interface {
 	Read([]byte) (int, error)
 }
 
+// NewKeccakState creates a new KeccakState
+func NewKeccakState() KeccakState {
+	return sha3.NewLegacyKeccak256().(KeccakState)
+}
+
+// HashData hashes the provided data using the KeccakState and returns a 32 byte hash
+func HashData(kh KeccakState, data []byte) (h common.Hash) {
+	kh.Reset()
+	kh.Write(data)
+	kh.Read(h[:])
+	return h
+}
+
 // Keccak256 calculates and returns the Keccak256 hash of the input data.
 func Keccak256(data ...[]byte) []byte {
 	b := make([]byte, 32)
-	d := sha3.NewLegacyKeccak256().(KeccakState)
+	d := NewKeccakState()
 	for _, b := range data {
 		d.Write(b)
 	}
@@ -74,7 +87,7 @@ func Keccak256(data ...[]byte) []byte {
 // Keccak256Hash calculates and returns the Keccak256 hash of the input data,
 // converting it to an internal Hash data structure.
 func Keccak256Hash(data ...[]byte) (h common.Hash) {
-	d := sha3.NewLegacyKeccak256().(KeccakState)
+	d := NewKeccakState()
 	for _, b := range data {
 		d.Write(b)
 	}