JournalingFileStore: move apply/commit sequencing to apply_manager

JournalingFileStore: move apply/commit sequencing to apply_manager

syncing the filestore requires a stable commit point (i.e., all ops
up to applied_seq must have been applied).  Previously, we used
journal_lock to atomically block new applies while waiting for
the remaining ones to finish.  This creates unnecessary contention.
We now use apply_manager to manage that state atomically with its
own lock.

Signed-off-by: Samuel Just <sam.just@inktank.com>