Further mockbrocker improvements

[horkhe]

This PR brings further improvements into the mockbrocker:

• A set of response builders were introduced to construct responses based on actual request data. Note that builders can respond to the most common request parameters. If something very specific is needed then a response can always be constructed directly;
• mockbroker.SetHandlerByMap method was introduced that provides a declarative way of defining a request handler that use mockresponse builders.
• consumer_test.go was rewritten using the new test primitives. Note that it is not a direct translation. Some tests were split for better isolation of concerns. Note also that TestConsumerRebalancingMultiplePartitions is using time delays for synchronization and therefore race prone. It could have been rewritten in almost non-racy way by removing the goroutine that reads from partition consumer channels and putting those reads among the lines of the main test flow. But I opted to leave it this way, because the goroutine code resembles how PartitionCosnumer is used in real life.

[horkhe]

@eapache it builds now that I rebased it on the latest master.

[eapache]

why does the test suddenly need so much more time?

[horkhe]

The consumer tests are rather fast, it is that other tests that use the older mock broker logic (the Returns() plus the default handler to be more specific) sometimes take longer to complete. The thing is that in the original mock broker implementation it would delay a response to a request indefinitely until a reply is provided via Returns() method. Now if a mock broker happens to get a request before Returns() is called it waits for expectationTimeout for Returns() to be called and then ignores the request making it IO timeout on the consumer side. Consumer retries eventually but that adds up to the execution time.

I will try to increase the expectationTimeout and bring the overall test suite timeout to the original value.

[eapache]

A few small points, but otherwise this looks really excellent, thank you!

[horkhe]

@eapache done

[eapache]

👍

@wvanbergen any comments?

[wvanbergen]

This looks good to me! Merging...