librbd: re-add support for nautilus clients talking to jewel clusters

librbd: re-add support for nautilus clients talking to jewel clusters

We want to support N - 3 client backward compatibility (special case
to support Jewel since it was a LTS release). The "get_snapshot_timestamp"
cls method does not exist in Jewel clusters so librbd should fallback
to excluding the op if it fails.

Note that this N - 3 also needs to apply for downstream releases as well,
which implies we still need Jewel for the time being.

Fixes: http://tracker.ceph.com/issues/39450
Signed-off-by: Jason Dillaman <dillaman@redhat.com>
(cherry picked from commit c644121820b83c97e68f9896393a45cd34787672)

Conflicts:
	src/test/librbd/image/test_mock_RefreshRequest.cc: tweaked to support pool configs
(cherry picked from commit d1a67c3bda09430bb21f768fb743b3e41c7d411e)