In this commit, we add the current chain parameters to the
BitcoindNotifier. This will be used in a future commit in order to
convert outputs scripts into addresses. This is needed since the
bitcoind backend uses these addresses to detect whether the script
encoded within it was spent by a transaction in the chain.
In this commit, we address a bug where it's possible that we still
attempt to manually scan for a transaction to determine whether it's
been included in the chain even after successfully checking the txindex
and not finding it there. Now, we'll short-circuit this process by
exiting early if the txindex lookup was successful but the transaction
in question was not found. Otherwise, we'll fall back to the manual
scan.
