In this commit, we modify the logic within RegisterSpendNtfn for the
BitcoindNotifier to account for the recent changes made to the
TxNotifier. Since it is now able to handle spend notification
registration and dispatch, we can bypass all the current logic within
the BitcoindNotifier and interact directly with the TxNotifier instead.
The most notable changes include the following:
1. We'll only attempt a historical rescan if the TxNotifier tells us
so.
2. We'll dispatch the historical rescan within the main goroutine to
prevent WaitGroup panics, due to the asynchronous nature of the
notifier.
