Removed assertion and specifically short-cutted the spawned_flush_ case in im_waiting_for_flush.

Only the assertion was bad, but the short-cutting behavior reduces the
"attack surface" of bugs on exists_flushable_txn_set et al.
Merge branch 'next' into sam_1990
Merge branch 'next' into sam_1990
Merge branch 'sam_1990' into next