lei up: automatically use dt: for remote externals
Since we can't use maxuid for remote externals, automatically
maintaining the last time we got results and appending a dt:
range to the query will prevent HTTP(S) responses from getting
too big.
We could be using "rt:", but no stable release of public-inbox
supports it, yet, so we'll use dt:, instead.
By default, there's a two day fudge factor to account for MTA
downtime and delays; which is hopefully enough. The fudge
factor may be changed per-invocation with the
--remote-fudge-factor=INTERVAL option
Since different externals can have different message transport
routes, "lastresult" entries are stored on a per-external basis.