MB-100: track 'Connection reset' in VBucketAwareMemcached

Change-Id: Ic69b4eaa43ac1f96cb0aff2d0b22a6cb0eae0014
Reviewed-on: http://review.couchbase.org/24969
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>