Like #4232, but for keypool.

Similarly to #234, it would be useful to provide functions for creating test KafkaProducers.

A good first function would be one which yields somewhat sensible default RecordMetadata.

object KafkaProducer {
  def unit[F[_], K, V](implicit F: Sync[F]): F[KafkaProducer[F, K, V]] = ???
}

Likely, this would require some internal state, hence F[KafkaProducer[F, K, V]].

Currently Travis doesn't provide openjdk 8 out of the box on bionic (see laserdisc-io/laserdisc#170). It's probably time to consider Circle CI (or even Github Actions) for the builds and investigate if it can solve the problem.

Sometimes it might be useful to know what event triggered what state. The internals of the various EventState implementations should be flexible enough to handle this option. In particular, this change would require rethinking the idea of an "initial state" to always require some incoming event. For the default case where there is no initial event, it can be modeled internally using Option[E]