rados/singleton/all/thrash_cache_writeback_forward_none: thrash cache pool drain

Enable/disable cache pool against a running workload.

Backport: firefly
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #44 from ceph/wip-7032

rados/singleton/all/thrash_cache_writeback_forward_none: thrash cache pool drain