In this commit, we introduce the registration logic for spend
notifications to the TxNotifier. Most of this logic was taken from the
different existing ChainNotifier implementations, however, it features
some useful additions in order to make the ChainNotifier a bit more robust.
Some of these additions include the following:
1. RegisterSpend will now return a HistoricalSpendDispatch struct,
which includes the details for successfully determining if an outpoint
was spent in the past. A HistoricalSpendDispatch will only be returned
upon the first registration of an outpoint. This is done as,
previously, if multiple clients registered for the same outpoint, then
multiple historical rescans would also be dispatched, incurring a toll
on the backend itself.
2. UpdateSpendDetails will now be used to determine when a historical
rescan has completed, no matter if a spending transaction was found or
not. This is needed in order to responsibly update the spend hints for
outpoints at tip, otherwise we'd attempt to update them even though we
haven't yet determined if they have been spent or not. This will
dispatch notifications to all currently registered clients for the
same outpoint. In the event that another client registers later on,
then the spending details are cached in memory in order to prevent
further historical rescans.
In this commit, we introduce the required fields for the TxNotifier to
properly carry its duties in notifying its registered clients about the
spend of an outpoint. These are not yet used, but will be throughout the
some of the following commits.
In this commit, we add a new channel within the SpendEvent struct that
will be sent upon whenever the spending transaction of the registered
outpoint gets reorged out of the chain. This will pave the road for
successfully handling a funding transaction getting reorged out of the
chain among other things.
