MB-7943: Log too many connections as warning level

Change-Id: I39cb3dd7314d95c52de11697d1499b3a69c324a5
Reviewed-on: http://review.couchbase.org/25710
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>