This changes the coordinator_id to be a unique string, e.g., `coordinator-1`, so that it will get a dedicated connection. This won't eliminate lock contention because the client lock applies to all connections, but it should improve in-flight-request contention.

The current client attempts to bootstrap once during initialization, but if it fails there is no second attempt and the client will be inoperable. This can happen, for example, if an entire cluster is down at the time a long-running client starts execution.

This commit attempts to fix this by removing the synchronous bootstrapping from `KafkaClient` init, and instead merges bootstrap metadata with the cluster metadata. The Java client uses a similar approach. This allows us to continue falling back to bootstrap data when necessary throughout the life of a long-running consumer or producer.

Fix #1670

If a valid broker in the cluster has no partitions, it will return None rather than an empty set.

Similarly updated a few other methods.