while we already refrained from propagating messages that would be
expunged from the target, we still propagated ones that would be
expunged from the source. this would lead to the weird situation of
creating orphans, and would pose journal replay idempotence problems.
such messages will now never have a sync record, so it becomes
pointless to test for S_PENDING in the trashing loop. note that the
behavior was previously bogus: these messages would have been paired by
the end of the run, so we shouldn't have treated them as solo for the
purposes of TrashOnlyNew/TrashRemoteNew.
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