admin_socket: add admin_stream

Add support for stream commands.  A stream hook provides an operator<<
interface for multicasting any input to all connected listeners.  A
user connects to a stream by using ceph command with the registered
command.

Signed-off-by: Samuel Just <sam.just@inktank.com>
OSD: make OpTracker dump ops to an AdminStreamHook in OSDService

Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG: dump push and pull events

Signed-off-by: Samuel Just <sam.just@inktank.com>