Before this commit, the only error returned from `IsOurTx` is when the
root bucket was not created. In that case, we should consider the tx to
be not found in our db, since technically our db is empty.
A future PR may consider treating our wallet as the single source of
truth and query the wallet instead to check for past sweeping txns.
This commit modifies the sweeper store to save a `TxRecord` in db
instead of an empty byte slice. This record will later be used to bring
RBF-awareness to our sweeper.
We remove the publishing of the last published sweep tx during the
startup of the sweeper. This republishing can lead to situations
where funds of the default wallet might be locked for neutrino
backend clients.
Moreover all related tests are removed as well.
