Use awaitility to read data in Kafka buffer tests to fix flakiness

[dlvenable]

Description

The current tests for KafkaBufferIT and KafkaBuffer_KmsIT wait on the buffer in a single call. These tests are flaky with Kafka 2.8.1. I found that there are transient errors happening here. But, 10 seconds may not be enough time for these errors.

So I've updated these tests to use awaitility. The tests can wait up to 20 seconds, but each call only waits half a second, so the tests are little faster overall.

I'm seeing that the tests against 2.8.1 are working well now.

Issues Resolved

Contributes toward #4168

Check List

• New functionality includes testing.
• New functionality has a documentation issue. Please link to it in this PR.
  • New functionality has javadoc added
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[dlvenable]

These changes resulted in passing builds locally and in my fork. However, now the tests are failing. Interestingly, the failures have moved further down the tests.

New failures:

KafkaBufferIT > writeBigJson_and_read() FAILED
    java.lang.AssertionError at KafkaBufferIT.java:220

KafkaBufferIT > write_and_read() FAILED
    java.lang.AssertionError at KafkaBufferIT.java:149

KafkaBufferIT > write_and_read_max_request_test() FAILED
    java.lang.AssertionError at KafkaBufferIT.java:190