osd/osd_types: add cache policy fields to pg_pool_t

Signed-off-by: Sage Weil <sage@inktank.com>
osd/ReplicatedPG: no HitSet on non-primary

Signed-off-by: Sage Weil <sage@inktank.com>
osd: add agent_tp, agent_wq

Separate work queue, thread pool for agent activity.

Signed-off-by: Sage Weil <sage@inktank.com>
wip