ISPN-2609 Infinispan SpringCache throws java.lang.NullPointerException:
Null values are not supported!

Add support for storing null values in and retrieving null values from a
SpringCache. Use solution provided by Roland Csupor as a starting point.

* Introduce NullValue, a singleton class that represents a null value in
a
cache.
* Make SpringCache use NullValue for representing null values in wrapped
Infinispan cache.
Remove unused class MembershipArithmetic
ISPN-2814 RpcManager doesn't return values properly

* Add proper wrapping of ReplicableCommand return values/exceptions into Response objects, to behave exactly as in case of CacheRpcCommands
* Add unit test using some custom commands to prove result wrapping works identically for CacheRpcCommands and ReplicableCommands in general
* CommandAwareRpcDispatcher and InboundInvocationHandlerImpl check if the result is already a Response to avoid double wrapping for legacy commands that do their own wrapping internally