This pull request introduces a mechanism to compress trienode history by
storing only the node diffs between consecutive versions.
- For full nodes, only the modified children are recorded in the history;
- For short nodes, only the modified value is stored;
If the node type has changed, or if the node is newly created or
deleted, the entire node value is stored instead.
To mitigate the overhead of reassembling nodes from diffs during history
reads, checkpoints are introduced by periodically storing full node values.
The current checkpoint interval is set to every 16 mutations, though
this parameter may be made configurable in the future.
This PR is the first step in the trienode history series.
It introduces the `nodeWithOrigin` struct in the path database, which tracks
the original values of dirty nodes to support trienode history construction.
Note, the original value is always empty in this PR, so it won't break the
existing journal for encoding and decoding. The compatibility of journal
should be handled in the following PR.
