MB-9825 Add more warning-level logs for disconnection events.

Change-Id: I3d8046c632e80cd01e86ce2cadb8e8c46aeba7a2
Reviewed-on: http://review.couchbase.org/32567
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>