Broker2Broker: Introduce Main Broker

When using Broker2Broker, there should be a Main Broker to which all other brokers connect.

This Main Broker should:

• display stats for the entire cluster, including total number of clients, executions, exec/secs, and objectives.
• for OnDiskCorpus, it should collect all the TestCases and objective outputs from all connected brokers.

This will make managing a multi-machine fuzzing campaign much easier.

Ideally, it should be possible to run a Main Broker without any clients of its own. It should also be possible to run Main Broker on a PC, while child-brokers and clients run on e.g. phones.