MB-100: enforce retry on every memached error in XMem

Original design will tolerate 10 memcached errors in a batch
without crashing and retry from earlier sequence number. The
intention was to prevent occasional memcached errors from
blocking replication at the cost that some items may not get
replicated.

By talking to support staff, this may not be the best way and
the top priority is to make data synced up on both sides without
missing any mutation. Therefore this fix will make replicator
retry on every memcached error.

Change-Id: Ia30b6e29ec3905f6d8ded8c461e010a2a35f2c90
Reviewed-on: http://review.couchbase.org/30995
Reviewed-by: Junyi Xie <junyi.couchbase@gmail.com>
Tested-by: Junyi Xie <junyi.couchbase@gmail.com>