Add CacheAdapterFactory to map interfaces to adapters

[schlessera]

The logic in ObjectCache is messy. Deciding what adapter to use should be done in a factory.

[felixarntz]

Again, just discovered this after working on https://github.com/felixarntz/wp-psr-cache/blob/master/src/CacheAdapter/PsrCacheAdapterFactory.php 😞

Let's compare the two approaches (although they seem rather similar). Any suggestions about mine?

[schlessera]

The main difference is that I created a mapping const, whereas you used conditionals.
You went with interface, which is always better. I didn't bother, as your previous code had mostly just fixed implementations.

[schlessera]

@tfrommen is right, accepting an FQCN string as interface does not make any sense here and could be removed.

[felixarntz]

I merged the change of using a mappings constant into the cache adapter I had implemented, including getting rid of the unnecessary checks in the foreach loop.