watch: avoid unnecessary spawning on spam removals
This should further mitigate lock contention problems
when -watch is configured to watch on a Maildir for spam
while performing a large NNTP import.
There is now a small risk a message won't get removed because if
it's in the current (uncommitted) fast-import batch, but
unlikely given the batch size is now only 10 messages.
If a that small window is hit, flipping the \Seen flag
(e.g. marking it unread, and then read again) will trigger
another removal attempt via IMAP or Maildir.