ReplicatedPG: disable clone subsets for cache pools

To do this properly, we need to take appropriate read
locks on clone sources and handle evicted clones.

Fixes: #7874
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #1553 from ceph/wip-7874

ReplicatedPG: disable clone subsets for cache pools

Reviewed-by: Sage Weil <sage@inktank.com>