Flip-6 client-cluster communication

With the new Flip-6 architecture, the client will communicate with the cluster in a RESTful manner.

The cluster shall support the following REST calls:

• List jobs (GET): Get list of all running jobs on the cluster
• Submit job (POST): Submit a job to the cluster (only supported in session mode)
• Lookup job leader (GET): Gets the JM leader for the given job
• Get job status (GET): Get the status of an executed job (and maybe the JobExecutionResult)
• Cancel job (PUT): Cancel the given job
• Stop job (PUT): Stops the given job
• Take savepoint (POST): Take savepoint for given job (How to return the savepoint under which the savepoint was stored? Maybe always having to specify a path)
• Get KV state (GET): Gets the KV state for the given job and key (Queryable state)
• Poll/subscribe to notifications for job (GET, WebSocket): Polls new notifications from the execution of the given job/Opens WebSocket to receive notifications

The first four REST calls will be served by the REST endpoint running in the application master/cluster entrypoint. The other calls will be served by a REST endpoint running along side to the JobManager.

Detailed information about different implementations and their pros and cons can be found in this document:
https://docs.google.com/document/d/1eIX6FS9stwraRdSUgRSuLXC1sL7NAmxtuqIXe_jSi-k/edit?usp=sharing

The implementation will most likely be Netty based.