osd: do not leak MOSDPings on shutdown

Signed-off-by: Sage Weil <sage@inktank.com>
osd: close classes on shutdown

Signed-off-by: Sage Weil <sage@inktank.com>
msgr: clear Connection priv on pipe shutdown

This breaks Connection <-> session struct reference loops so that both get
cleaned up when the Pipe shuts down.

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