osd/PeeringState: give PeeringCtxWrapper a BufferedRecoveryMessages ref

osd/PeeringState: give PeeringCtxWrapper a BufferedRecoveryMessages ref

The ref can be either a raw BufferedRecoveryMessages (buffering) or
a PeeringCtx (child class, no buffering).  This avoids the need for a
member ref, and also means that we can (later) make methods take the
BufferedRecoveryMessages ref as a general interface to consume for both
buffered and non-buffered work that is outside of PeeringState.

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