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>
Merge remote-tracking branch 'membase/branch-20'

* membase/branch-20:
  MB-7943: Log too many connections as warning level

Change-Id: I05e6c4cadc6e08c56c99e104ba4fc0a1e45de223