The new Broker API allows us to provide a custom logger to Broker that
pulls previously unattainable context information out of Broker to put
them into broker.log for users of Zeek.
Since Broker log events happen asynchronously, we cache them in a queue
and use a flare to notify Zeek of activity. Furthermore, the Broker
manager now implements the `ProcessFd` function to avoid unnecessary
polling of the new log queue. As a side effect, data stores are polled
less as well.
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