Sqs-workers

- Updated README with examples on how to process a dead-letter queue in-place
- Added a way to add tasks in batch

Changed

- Declared support for Python3.8
- Made sqs-workers work better in Python2 / Python3 mixed environment

Changed

- Deprecated calling AsyncTask(**kwargs). Now, if you want to run it
synchronously, you should use AsyncTask.run(**kwargs) instead.
Use AsyncTask.delay(**kwargs) as always to add the task to the queue.

Changed

- Removed support for batched processors
- On top of existing sqs.processor("queue", "job_name") added
sqs.raw_processor("queue") to process unstructured messages from the qeueue
- Extracted functions to create and delete queues for SQSEnv to a separate module
- Remove most of the "proxy methods" for SQSEnv: purge_queue, drain_queue,
process_queue, process_batch, get_raw_messages. SQSEnv.queue() has to be used
instead.

Added

- Added support for baked async tasks (async tasks + arguments)

Fixed

- Fixed DeadLetterProcessor with FIFO queues

Changed

is_deadletter() and get_deadletter_upstream_name() extracted from
methods to independent functions.

Added

- Added support for implicit context for tasks
- Added ASAP_SHUTDOWN shutdown policy to stop processing of the queue
as soon it's found empty

Fixed

- Fixed batch processor which used to reorder messages from FIFO queues

Changed

Made a refactoring of SQS environments and extracted in two separate classes
ProcessorManager and ProcessorManagerProxy.

It's changed the API of SQSEnv, so that instead of
`sqs.connect_processor(queue, 'say_hello', say_hello)` the API of
`sqs.processors.connect(queue, 'say_hello', say_hello)` has to be used.

Also overwriting "makers" after the initialization of SQSEnv needs to be done
in a different way. Instead of `sqs.fallback_processor_maker = DeadLetterProcessor`
the syntax of `sqs.processors.fallback_processor_maker = DeadLetterProcessor`
should be used instead.