PLATFORM-575: Add support for memcached in DPL

Bring back DPL cache in memcached. It was accidentally removed when DPL was upgraded.
Original commit: ae9d0670baff9833ec

https://wikia-inc.atlassian.net/browse/PLATFORM-575

/cc @macbre @michalroszka 
``wfMemcKey`` performs str_replace internally.