allow configuration of memcached options (#23729)

* allow configuration of memcache options

Use production values for memcached as explained in http://apprize.info/php/scaling/15.html

The current implementiation uses ascii based serialization. This PR should reduce traffic to the memcached server.

cc @MorrisJobke @FelixBoehm

* add config sample

* merge config options, throw hint on config error

* fix typo

* fix config sample