PROV-1070 Rewrite MemoryCache

We now use a custom implementation based on PHP arrays since we removed
Zend\Cache. Also simplified interface a little bit. We probably won’t
need these multi-item access functions.

Also remove ExternalCacheTest because we’re going to rewrite that as
well.
PROV-1070 Refactor MemoryCache to mimic Doctrine cache interface

Having the same interface for both internal and external caching should
help our sanity.