[trie] Separate sync mode and async mode in Trie

[Liuhaai]

What would you like to be added: Create branchnode_sync.go, leafnode_sync.go, extensionnode_sync.go under ./db/trie/mptrie

Why is this needed: We switched trie from sync mode to async mode by a hardfork. Currently, the sync mode is only useful for blks below the hardfork height. Separating them from current code will improve code readability.