osdc/Objecter: pass correct cookie value to error

Signed-off-by: Sage Weil <sage@redhat.com>
ceph_test_rados_api_watch_notify: weak assert cookie is valid (it's a ptr)

Signed-off-by: Sage Weil <sage@redhat.com>
ceph_test_rados_api_watch_notify: wait longer for osd to time us out

If cluster is thrashing this can fail.

Signed-off-by: Sage Weil <sage@redhat.com>
osdc/Objecter: only issue one error per watch; do it through one path

Send all errors through C_DoWatchError for simplicity.

Only deliver one error per watch.  This simplifies callers logic and avoids
a very rare case that they need to deal with.

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