crimson: hook PeeringState into pg

crimson: hook PeeringState into pg

This patch:
1) Adds partially implemented stubs for PeeringState::PeeringListener
2) Revamps the message handling to batch the peering events from
   a single message.
3) Replaces the message handling with hand-offs to PeeringState
4) Adds support for MOSDPGCreate2 enabling creation of pgs.
5) Introduces ShardServices for exposing osd machinery to the pgs.

Signed-off-by: Samuel Just <sjust@redhat.com>