"Fixes: #469
We occasionally generate memtables that are not empty, yet have no
high replay_position set. (Typical case is CL replay, but apparently
there are others).
Moreover, we can do this repeatedly, and thus get caught in the flush
queue ordering restrictions.
Solve this by treating a flush without replay_position as a flush at the
highest running position, i.e. "last" in queue. Note that this will not
affect the actual flush operation, nor CL callbacks, only anyone waiting
for the operation(s) to complete.
To do this, the flush_queue had its restrictions eased, and some introspection
methods added."
cf734132e7