Extend Cache API to support entries existence check

Cache API should allow to check if an entry exists.

Using the get API could be an option, but :

• it is more expensive than actually needed
  • ex: Redis marshaling
• it may change the internal cache state
  • ex: modify the internal "last access time" that can be used for LRU

We should add an hasEntry API.
For compatibility reasons, the abstract class will provide an implementation based on the get API.