crimson/osd: introduce OpsExecuter to uniform calling CEPH_OSD_OPS.

crimson/osd: introduce OpsExecuter to uniform calling CEPH_OSD_OPS.

OSD has two entry points for executing CEPH_OSD_OP_*:
  1. the MOSDOp message handler,
  2. the Object Class API (cls_* and cls_cxx_* functions).

We definitely want to address these two users without code
duplication. However, exposing the entire PG to Obj Class
would break encapsulation. Moreover, there is difference
in life times between PG and sequence-of-operations-from-
MOSDOp.

Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>