Manual Broker Retry

Allow the broker to be invoked manually via a policy callback:

    razor policy callback <active model> broker

This will trigger the broker_agent_handoff method on the policy. It
takes no arguments and simply restarts the broker handoff process. This
may be useful for testing out brokers or retrying a failed broker.

Fixes #420
Merge pull request #438 from calebcase/feature/manual-broker

Manual Broker Retry