Allow to use hash() instead of __ring_key__ for in-memory storage

[youknowone]

Because hash() returns diffrent values by processes, it should be used only for in-memory storages.
Still, in-memory cache is a big part of use case of ring.

Implementation note:

• Add in-memory storage marker to storage.
• Fix coerce() function to receive whether hash() is allowed or not.
• The priority is:
  • Call ring_key first
  • Fallback to hash() if it is allowed
  • Warning if nothing available