osdc/Objecter: make watch_check time reflect async delivery

osdc/Objecter: make watch_check time reflect async delivery

When a librados user calls watch_check(), the age they get back should
establish a timestamp for which:

 1) we know that the watch was registered at least through this time,
 2) we have received and processed notifies sent as of this time,
 3) we have processed any async error events as of this time.

We already accomplish 1 by updating watch_valid_thru based on the time
the ping is sent (not received).

This patch gets us 2 and 3 by using the MIN of watch_valid_thru and the
oldest queued async event for the watch_check result.

Signed-off-by: Sage Weil <sage@redhat.com>