Watch/Notify: rework watch/notify

Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG: accept watch cookie value with notify ack

Signed-off-by: Samuel Just <sam.just@inktank.com>
librados/: include watch cookie in notify_ack

Signed-off-by: Samuel Just <sam.just@inktank.com>
doc: add some internal docs for watch/notify

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