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.
