osdc: Use a finisher from Journaler

osdc: Use a finisher from Journaler

Completions from I/O operations (i.e. the objecter) hop
through the finisher twice, because of the three layers of
locking (MDS::mds_lock -> Journaler::lock -> Objecter osd session lock)

Because on the way "right" we take the locks in that order, to avoid
deadlock we can't take the locks in the opposite order on the way
"left", hence the finishers.

Signed-off-by: John Spray <john.spray@redhat.com>