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

I noticed during local testing that version probing was happening twice when connecting to newer broker versions. This was because we call check_version() once explicitly, and then again implicitly within get_api_versions(). But once we have _api_versions data cached, we can just return it and avoid probing versions a second time.