Remove allowExperimentalV011 flag

[Nevon]

This has been defaulted to true for almost two years, so it's hardly experimental anymore.

src/protocol/requests/index.js is where the meat of the change is. Essentially if the flag was false, we'd filter out any api version above a certain limit for Fetch and Produce requests.

[Nevon]

Hmm, so this PR reveals a bit of nastiness. The tests that are currently failing are ones where allowExperimental011 was false. They now fail because we are returning some new data in the responses, which is normally fine except that some of that is from producer.send, which is a public API...

For example, a response when allowExperimental011 is false:

{
  "errorCode": 0,
  "offset": "0",
  "partition": 0,
  "timestamp": "-1",
  "topicName": "test-topic-aa740d312ca4c5c8d606-75297-2699790b-a3f0-4706-891c-7e7c2b410bc4",
}

And when it's true:

{
  "baseOffset": "0",
  "errorCode": 0,
  "logAppendTime": "-1",
  "logStartOffset": "0",
  "partition": 0,
  "topicName": "test-topic-aa740d312ca4c5c8d606-75297-2699790b-a3f0-4706-891c-7e7c2b410bc4",
}

Our type definitions say that the response the following, which is just a flat out lie for Kafka >=0.11:

type RecordMetadata = {
  topicName: string
  partition: number
  errorCode: number
  offset: string
  timestamp: string
}

[Nevon]

Updated the type definition of RecordMetadata to not be a lie anymore.

This also revealed a problem in how some of our tests were structured. Basically, any test that doesn't use a testIfKafka<version> function assumes that it will pass in any version of Kafka. For testing 0.11 or 1.1 features, we have testIfKafka_0_11 and testIfKafka_1_1_0. Now this assumption isn't right anymore, because we now have some tests that will pass in 0.10 but not in >0.10. So I had to create testIfKafka_0_10 - however, unlike the other ones, this one will only run if the Kafka version is less than or equal to 0.10. This felt weird, so I renamed them testIfKafka<AtMost|AtLeast>_<version>.

[ankon]

... RecordMetadata ... types ...

This somehow triggered something in my brain: You may have accidentally also fixed or improved #464 now :)

[Nevon]

Oh wow, I had completely forgotten about that. The new RecordMetadata type is now essentially an API version agnostic response type. However, I still think we should make that explicit at runtime (there's a convenient serializeResponses function where we could do it), but I'll leave that out of this PR, as it's already grown too much.