osd/osd_types: add cache policy fields to pg_pool_t

Signed-off-by: Sage Weil <sage@inktank.com>
mon/OSDMonitor: allow new pool policy fields to be set

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