osdc/Journaler: use finisher for public callbacks

osdc/Journaler: use finisher for public callbacks

This is needed because of occasional lock cycles with
external callers doing e.g. write_head.

We do get some weird-looking multiply-nested
C_OnFinisher(C_OnFinisher(...)) from this approach,
where one finisher exists to protect journaler from
lock cycles wrt objecter, and the other exists
to protect the MDS from lock cycles wrt journaler.

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