crimson: update osd when peer gets authenticated

crimson: update osd when peer gets authenticated

* common/auth_handler.h: add an abstract class of AuthHandler, the one who is interested in
  an authenticated peer should implement this class
* mon/MonClient: let mon::Client implement AuthServer, as it has access the keyring. it
  will update the registered AuthHandler if the client (peer) is
  authenticated.
* osd: implement AuthHandler class. we will keep track of the connected
  sessions along their caps in a follow-up change.

Signed-off-by: Kefu Chai <kchai@redhat.com>