zcash_client_backend: Periodically update the recover-until height for improved progress reporting.

[nuttycom]

After #1541, we likely want to periodically bump the recover-until height when we detect we've been offline for long enough that older history is technically recovery. @nuttycom reminded me that the ideal criterion for this is "the chain tip is in a new subtree", because that means we no longer need to scan every note in the old subtree in order to make existing notes spendable (we'll instead grab its subtree root).

Originally posted by @str4d in #1070 (comment)