Align note commitment ranges for subtree construction to subtree boundaries

[nuttycom]

In https://github.com/zcash/librustzcash/blob/main/zcash_client_sqlite/src/lib.rs#L1175, we set and use a fixed power-of-two chunk size for note commitment tree subtree construction. A minor performance improvement would be to use the power-of-two boundaries in the note commitment tree to make it so that insertion of a subtree can consist of insertion of just a single node in the circumstance that there are no marked (or checkpointed) children. This may require this calling code to avoid creating checkpoints except within 100 blocks of the current chain tip; we should make sure we're not creating excessive checkpoints anyway so that we minimize the amount of pruning that is required.