crimson/.../ops_executer: rework prepare_transaction/mutate_object

crimson/.../ops_executer: rework prepare_transaction/mutate_object

That the log entry's verison matches the object_info on the actual
object is a pretty core invariant.  This commit moves creating the
log entry for head and populating the metadata into
OpsExecuter::prepare_head_update.

As a side effect, flush_clone_metadata and CloningCtx::apply_to
were removed and split between prepare_head_update (portions
related to the head's ssc) and flush_changes_and_submit.

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