mds: preserve ESlaveUpdate::OP_PREPARE logevent before doing commit

mds: preserve ESlaveUpdate::OP_PREPARE logevent before doing commit

Fixes: https://tracker.ceph.com/issues/45024
Signed-off-by: songxinying <songxinying@sensetime.com>
(cherry picked from commit 4940ab62e0d19ce36e53bcc67b2a2161c47f6c6d)

Conflicts:
    src/mds/MDCache.cc
- use MMDSResolve::create() in nautilus, instead of make_message<MMDSResolve>()
    src/mds/MDCache.h
    src/mds/Mutation.h
- in nautilus, these two files are structured differently from master (large
  chunks of the master code are missing in nautilus, ordering of code is
  different also)
    src/mds/Server.cc
- use nautilus equivalent instead of "make_message<MMDSSlaveRequest>"