In this commit, we fix a recently introduced bug that would cause
callers that attempted to get a spend notifications for a particular
outpoint to no longer get that precise notification. In a prior commit,
in preparation for the new neutrino gcs filter format, we added a
mapping from script to outpoint that will be used by neutrino, and later
other backends as well. However, we still need to match on outpoint
slices for bitcoind today.
This commit also allows bitcoind back-end to return block timestamp
in `BlockStamp()` and prevents logging and notification of events
until after the client wallet requests notifications.
