Ceph

objecter: always resend linger registrations

If a linger op (watch) is sent to the OSD and updates the object, and then
the client loses the reply, it will resend the request. The OSD will see
that it is a dup, however, and not set up the in-memory session state for
the watch. This in turn will break the watch (i.e., notifies won't
get delivered).

Instead, always resend linger registration ops, so that we always have a
unique reqid and do the correct session registeration for each session.

• track the tid of the registation op for each LingerOp
• mark registrations ops as should_resend=false; cancel as needed
• when we send a new registration op, cancel the old one to ensure we
  ignore the reply. This is needed becuase we resend linger ops on any
  pg change, not just a primary change.
• drop the first_send arg to send_linger(), as we can now infer that
  from register_tid == 0.

The bug was easily reproduced with ms inject socket failures = 500 and the
test_stress_watch utility.

Fixes: #2796
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>