Opening this Jira to track https://github.com/apache/kafka/pull/6218, since it's a rather subtle change.
In some test cases it's desirable to instantiate a subclass of `ShutdownableThread` without starting it. Since most subclasses of `ShutdownableThread` put cleanup logic in `ShutdownableThread.shutdown()`, being able to call `shutdown()` on the non-running thread would be useful.
This change allows us to avoid blocking in `ShutdownableThread.shutdown()` if the thread's `run()` method has not been called. We also add a check that `initiateShutdown()` was called before `awaitShutdown()`, to protect against the case where a user calls `awaitShutdown()` before the thread has been started, and unexpectedly is not blocked on the thread shutting down.