When scanning the history of the Federation, there’s no need to be
so cautious about the privacy (as it’s perfectly private), so might
as well increase the gap limit.
We will check this many addresses after last actually used
one before we give up
New client will start deriving new addresses from last used one
plus that many indexes. This should be less than
MAX_GAP, but more than 0: We want to make sure we detect
deposits that might have been made after multiple successive recoveries,
but we want also to avoid accidental address re-use.