Homogenization of multi-key support in Cache 

This PR adds support for multi-key read/write/delete to all cache adapters. With this change multi-keys usage can _always_ be used and relied upon as this is now a required feature of cache adapters.

Note: Userland Cache Adapters must be adapted to the changes to support multi keys. 

This PR also fixes a few inconsistencies with storing and retrieving `null` values.