Allow registering more cache adapters

[felixarntz]

Allow registering more cache adapters

[felixarntz]

It is now possible to register additional cache adapters, to be used for certain groups. These are still kept separated between persistent and non-persistent, as there is still a distinction to be made (whether a group is persistent or non-persistent must be determined before checking which cache adapter to use for a specific group).

This functionality was made possible by a bit of refactoring: The responsibility for cache adapters now fully belongs to the CacheSelector. Therefore, the main ObjectCache class no longer holds any cache implementations, they are instead part of CacheSelector which in turn is now the main parameter for ObjectCache. Adjustments in the ObjectCacheFactory and introduction of a new CacheSelectorFactory maintain simplicity of setting up the object cache.

Additional cache adapters per group can be registered via wp_object_cache()->getSelector()->registerPersistentCache($cache, $groups) or wp_object_cache()->getSelector()->registerNonPersistentCache($cache, $groups).