Use memory pool when encoding and hashing the trie.

[arijitAD]

Issue summary

• Use memory pool when encoding and hashing the trie. Also, we can parallel hash the nodes.

Other information and links

• Geth does the same.
• https://github.com/ethereum/go-ethereum/blob/7a7abe3de84d8f2c6b591fec11d67edcb82ae63d/trie/hasher.go#L47
• https://github.com/ethereum/go-ethereum/blob/7a7abe3de84d8f2c6b591fec11d67edcb82ae63d/trie/hasher.go#L123