zcash_client_backend: Make it possible to skip note commitment tree updates except in shards where one of our notes is found.

[nuttycom]

The following workflow is possible: under ordinary conditions, do not update the note commitment tree. When a note is found, the FoundNote scan range should include a request that the client fetch the tree state as of the end of the block prior to that containing the note. Then, the client can insert the tree state directly into the shard tree instead of for all notes prior to the note within the shard, and scan_cached_blocks can be modified to update the note commitment tree only when scanning FoundNote or ChainTip ranges.