Create abstraction for Cache

[AnesBenmerzoug]

We should create a proper abstraction of the caching to make the code base cleaner and to allow alternative implementations.

We currently only support Memcached.

This is related to issue #124

[mdbenito]

Additionally, the lifecyle of the cached objects is difficult to understand / opaque. And because we pass cache configs and the memoization decorator connects and holds the client, it is awkward to flush the cache. A trivial stopgap with a neat syntax would be add a property to decorated objects to allow cache flushing.

[AnesBenmerzoug]

Consider writing a notebook about caching.