Add implementation for Memcached

[Philipp Gillé]

Memcached is meant to be used as a cache and not as persistent key-value store, but some people might still like to use the simple gokv interface for using Memcached, or maybe they have configured persistency (despite Memcached discouraging to do this).

Also, some other databases are compatible with the Memcached protocol, for example Couchbase (see here) and Apache Ignite (see here).

• https://memcached.org/
• https://github.com/memcached/memcached
• https://github.com/bradfitz/gomemcache
• https://godoc.org/github.com/bradfitz/gomemcache/memcache

[Philipp Gillé]

Note: The gomemcache package seems to only implement the text protocol, while some databases that are compatible with the Memcached protocol mean the binary protocol.