core, ethdb, tests, trie: introduce database snapshot

[rjl493456442]

This PR introduce a new API for ethdb, the database snapshot.

It can happen that we want to get the specified version of the data from the database,
to be more specific, retrieve the data from the specified disk snapshot.

Right now this feature is unneeded since basically all data in disk have a unique identifier.
However, when we switch to path-based state scheme, this feature is required, for example:

• we want to trace block at height N, the disk state is at N + 100
• we need to rewind the disk state to height N-1 by applying the reverse diffs
• In the same time, the disk state is mutated by chain by pushing the new version state
• the trace state has these requirements:
  • state reverts should not affect the live state
  • the live state changes should not affect the trace state

In order to achieve these requirements, we can build a mirror disk state with disk snapshot,
apply the reverse diffs on the top, save these reverts in the disk temporary area, to provide
a consistent view of state and eventually drop all the temporary state reverts and release
the disk snapshot when tracing is finished.

This PR intents to be merged separately.

[holiman]

LGTM