In this commit, we modify all existing historical rescans for
ChainNotifier backends to scan backwards rather than forwards. If we
know that a transaction has been confirmed, or outpoint spent, the it's
likely that the event has recently transpired assuming we've been
offline for a short period of time. Therefore, if we scan backwards
rather than forwards, then we can save potentially hundreds or thousands
of block fetches if the event recently happened close to the tip of the
chain.
We bound this search at the genesis block, to ensure we don't underflow
the uint32 used throughout the package in the main loop.
28eb8474f4