extend SQS event handler

[eviltwin]

Description

• report_batch_item_failures for partial batch processing
• maximum_concurrency to provide limits to SQS event source scaling
• remove outdated documentation about FIFO queues being unsupported

[eviltwin]

It was quite a small extension of the existing SQS handler, so I wasn't sure how much documentation would need adding for it... I'm happy to elaborate more on what I put, but the existing options didn't come with much explainer so I wasn't sure if that was a deliberate choice or not.

I corrected the FIFO docs part mostly in passing (I have confirmed that a FIFO queue works just fine).

[monkut]

It looks like we don't have good test coverage around this, but can you try to add testcases to cover this addition?

It appears we have minor coverage in test_placebo.py has event_source related testcases.

[coveralls]

coverage: 74.655% (-0.2%) from 74.81%
when pulling 61f8407 on eviltwin:master
into a38058b on zappa:master.

[eviltwin]

@monkut thanks, I'd looked for the test coverage when SQS events were originally implemented but I couldn't find them. I'll try to find the time in the next week to add coverage in the places you indicated :)