Acquire RM leadership before registering with Mesos

Mesos doesn't support fencing tokens in the scheduler protocol; it assumes external leader election among scheduler instances. The last connection wins; prior connections for a given framework ID are closed.

The Mesos RM should not register as a framework until it has acquired RM leadership. Evolve the ResourceManager as necessary. One option is to introduce an ResourceManagerRunner that acquires leadership before starting the RM.