osd/: use PGBackend::call_write_ordered to submit log entries in commit order

osd/: use PGBackend::call_write_ordered to submit log entries in commit order

Without this change, we might submit new log entries for marking objects
unfound in a way that causes replicas to process them out-of-order with
pending writes with lower version numbers.  That would be bad.  Instead,
add an interface to allow an arbitrary callback to be called after any
previously submitted transaction commit, but before any subsequently
submitted operations commit.

Signed-off-by: Samuel Just <sjust@redhat.com>