so far, we checked for M_DEAD only in loops over messages. but we should
have checked srec->msg uses as well. this would make the code a mess, so
instead call back from the drivers when messages are expunged, so we can
reset the pointers.
the only case where this really matters so far is the flag setting loop,
which may cause the concurrent expunge of not yet handled messages to be
detected by the maildir driver.
this tests only the common case of the far side being async - adding
100% instead of 50% to the runtime of the test to cover a corner case
didn't seem worth it.
