In this commit, we add a new Updates channel to our ConfirmationEvent
struct. This channel will be used to deliver updates to a subscriber of
a confirmation notification. Updates will be delivered at every
incremental height of the chain with the number of confirmations
remaining for the transaction to be considered confirmed by the
subscriber.
All implementations of the ChainNotifier interface support registering
notifications on transaction confirmations. This struct is intended to
be used internally by ChainNotifier implementations to handle much of
this logic.
