crimson/osd: sending EVENT_DISCONNECT becomes implementation detail of Watch.

crimson/osd: sending EVENT_DISCONNECT becomes implementation detail of Watch.

In contrast to ceph-osd crimson sends CEPH_WATCH_EVENT_DISCONNECT directly
from the timeout handler and after CEPH_WATCH_EVENT_NOTIFY_COMPLETE.
This simplifies the Watch::remove() interface as callers aren't obliged
anymore to decide whether EVENT_DISCONNECT needs to be send or not -- it
becomes an implementation detail of Watch.

Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>