KAFKA-18794: Add check to send one ShareFetchEvent per poll in ShareConsumer.

[ShivsundarR]

What

• Currently in ShareConsumerImpl, there is a chance we could be
  sending multiple ShareFetchEvents for a single poll().

• This could happen when the ShareFetchBuffer is empty in the first
  wait and the pollTimer has not completed yet, then we would send
  multiple events.

• Usually the ShareFetchBuffer would wait for a time until the
  pollTimeout, so we would not send multiple events, but the buffer wait
  time is also controlled by applicationEventHandler.maximumTimeToWait()
  which can return lower values (even 0) in some cases (especially during
  startup of heartbeat request manager).

• If this happens, we will see multiple events sent, and this could even
  lead to multiple fetching from the broker (sort of a fetch and a
  pre-fetch) if the response for the previous ShareFetchEvent arrives
  before the next ShareFetchEvent was processed.

• To avoid this, we have a check now which only sends one
  ShareFetchEvent per poll.

• This was the reason a couple of tests were flaky in
  KafkaShareConsumerTest -
  https://issues.apache.org/jira/browse/KAFKA-18794. The PR fixes the
  flakiness, now the tests reliably pass locally.

Reviewers: Andrew Schofield [email protected]

[AndrewJSchofield]

Just one trivial comment but we must maintain the standards :)

[AndrewJSchofield]

nit: The tests should start with test.

[ShivsundarR]

Have updated it now, thanks :))