Enable fast-update of witness data for existing notes (Spend-before-Sync)

[str4d]

This is an intermediate step on the way to a full DAGSync architecture. This issue can be closed when we have wallet datastores and APIs that can enable existing notes to be made spendable without calling scan_cached_blocks. The workflow will be something like:

• Record new-style tree data for new notes.
• Roll this tree data forward via range updates of historic tree data.
• Scan the last 100 blocks (or last 2^16 tree or whatever) to fill in the remainder.

[nuttycom]

I've added #404 as a blocker for this ticket because even if we don't complete orchard integration as part of this ticket, we need to do all of the conceptual work for how to support Orchard in the data access API at the same time or else we're going to end up doing a lot of needless extra work. We don't have to actually implement Orchard, but we need the API updates to make it so that we don't have to rewrite the data access API again when we do implement Orchard.

[nuttycom]

Closing as complete.