Harden Lambda FIFO integration tests.

[ashovlin]

Issue #, if available: N/A, follow-up to #71

Description of changes: @96malhar pointed out that the new Lambda FIFO integration test failed on his PR #77. Ran them continually locally, and saw failures in the 5-10 iteration range.

I'm now able to get to 10 iterations without failures (at which point I stopped) with these changes:

• For the FIFO messages logged to CloudWatch by the Lambda, we sort based on Timestamp instead of IngestionTime. I believe that is more accurate for when the Lambda function handled the message.
• Increased the period we're waiting for the logs from 15 to 20 seconds. Saw an iteration where there were 0 messages in the logs at the point we tried to assert them.

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