filestore: make OpSequencer::flush() work for writeahead journaling items

filestore: make OpSequencer::flush() work for writeahead journaling items

It was only waiting for items in the op_queue to complete.  The goal is
to wait for anything we've called queue_transactions(&osr,...) on. If we
do writeahead journaling, though, there might be new ops that are still
journaling but not yet submitted to the fs that are missed.

This adds a journal queue to the OpSequencer, and uses it in the writeahead
case only.

Signed-off-by: Sage Weil <sage@newdream.net>