Disengaging imsgev is tricky. First imsgev_clear as we know there
isn't anything left to be written. Signal imsgev termination by
setting iev->terminate = 1(Mind that we are setting it from with
the callback). As imsgev would again callback dispatch with
IMSGEV_DONE, we cannot free imsgev yet. So iev_maildrop need to
exist beyond lifetime of session until IMSGEV_DONE. Allocating it
seperately from session.
imsgev_close is avoided as it schedules another EV_WRITE which is
not needed in our case.
This fixes a crash observed consistently with a little perl script
to simulate concurrent sessions. Thanks MALLOC_OPTIONS='SFG<<'