MODE-2056 Updated the REST service so that any session used for servicing a request is always logged out at the end, either in the happy or the exceptional cases.

To support this, a couple of other changes were made:
- the modeshape-api/Repository interface was updated to expose the current active sessions
- the modeshap-api/Logger interface was updated and the standard  "isEnabled" methods added
- a Logging interceptor was added which, in DEBUG mode, can output various information about the incoming requests.