Remove gxo_sessions bundle/package. Everything it did was already done by GxoServerImpl too.
Remove MethodInvocationChecker, and introduce RemoteCallValidator.

MethodInvocationChecker was gxo-specific, while support for
RemoteCallValidator can be implemented by any RPC mechanism.
Merge pull request #18 from gevaerts/master

Replace MethodInvocationChecker by more generic RemoteCallValidator