core/state: introduce stateupdate structure

[rjl493456442]

This pull request rewrites the statedb by introducing a state update struct.

A state update object will be constructed once the statedb.Commit is invoked. It contains all the state
changes, e.g.

• original state root and post-execute state root
• deleted accounts along with their original value
• updated accounts along with their original value
• dirty contract codes
• dirty trie nodes (state hasher)

Instead of committing the state change into state snapshot and trie database blindly, the state update
can be consumed by different state database implementation in different ways. e.g.

• state update can be ignored if it's a read-only statedb
• flat state changes can be ignored if it's a trie-only statedb
• trie changes can be ignored if it's a flat-state only statedb
• etc

What's more, the statedb has been simplified significantly, especially in flat state change construction.