librados: define updated watch/notify interface

- new notify callback with the correct values:
  - notify_id
  - watch handle
  - payload
- new notify_ack call
  - not implicit when the callback returns (for new api only)
  - optional payload
- new watch2 call
  - that provides the new callback
- new notify2 call
  - with the right arguments, and optional timeout

A couple refactors in here:
- IoCtx notify_ack is now called unlocked (Note: this will soon change
  with pending Objecter locking changes)
- Objecter notify_ack takes a buffer

TODO:
- no timeout on the individual watch, yet...

Signed-off-by: Sage Weil <sage@redhat.com>
osd: implement notify ack payloads

If the notified send back reply payloads, pass them back to the notifier.

Note that we have changed the on-wire behavior of the watch completion
message a bit: instead of sending the original notify payload back to the
notifier, we send the map of notified to replies.  Note that only users of
the new API will know what to do with the notify acknowledgement
information.  At the same time, we stop sending the original payload.
However, the old API users never saw that data; we were uselessly sending
it over the wire.

Signed-off-by: Sage Weil <sage@redhat.com>
librados: update notify2 API to accept reply payloads

Allow the notify2 callers to provide bufferlists or char ** pointers
so that we can pass the reply buffer back to them.

Signed-off-by: Sage Weil <sage@redhat.com>
ceph_test_rados_api_watch_notify: test notify2 api

Signed-off-by: Sage Weil <sage@redhat.com>
librados: include notifier's gid in notify

The notify replies include the identify of everyone who acked the
notify.  For symmetry, inform the notified client who sent the notify.

Fixes: #9198
Signed-off-by: Sage Weil <sage@redhat.com>
osd: allow multiple watchers/notify replies per client

It is perfectly valid to register multiple watchers via the same
librados client.  Make the notify_replies a multimap to capture all
of the acks.

Signed-off-by: Sage Weil <sage@redhat.com>
librados: unconditionally pass WATCH_NOTIFY to legacy callback

This was always WATCH_NOTIFY; just pass that explicitly.  No need to
pay attention to what's in the message.

Signed-off-by: Sage Weil <sage@redhat.com>
osd: assume WATCH_NOTIFY for MWatchNotify

Signed-off-by: Sage Weil <sage@redhat.com>
librados: define CEPH_OSD_WATCH_OP_*

Signed-off-by: Sage Weil <sage@redhat.com>
librados: use new watch op codes; simplify Objecter helpers

- drop the useless add_watch() helper; do it explicitly
- drop the unused var arg everywhere
- make a separate notify member of the union that excludes
  the other unused fields

Signed-off-by: Sage Weil <sage@redhat.com>
osdc/Objecter: fix assert_src_version

Don't use the add_watch() helper; do it like assert_version
does it.

Signed-off-by: Sage Weil <sage@redhat.com>
squashme: add reconnect op
wip