[athena] fix TypeError while delete messages from SQS

[chunyong-lin]

to: @jacknagz or @ryandeivert
cc: @airbnb/streamalert-maintainers
size: small
resolves #590

Background

We noticed there is Athena error due to TypeError while deleting messages from SQS. The root cause of TypeError is because of complicated list comprehension logic when first deletion failed. The list comprehension added extra list around pushed back messages.

Changes

• Break down complicated list comprehension to simpler ones.
• Add mock class for SQS client and add unit test case to reproduce error.
• Add a max_retries in backoff logic to make the code testable.

Testing

• Rule testing

python manage.py lambda test --processor all
...
StreamAlertCLI [INFO]: (60/60) Successful Tests
StreamAlertCLI [INFO]: (93/93) Alert Tests Passed
StreamAlertCLI [INFO]: Completed

• Unit testing

./tests/scripts/unit_tests.sh
...
Ran 530 tests in 8.466s

OK

[coveralls]

Coverage increased (+0.07%) to 95.469% when pulling ecab817 on fix_sqs_delete_type_error into 954b70b on master.

[jacknagz]

Looks great, thanks for the very informative PR body too!