Add support for async I/O

[sethmlarson]

• Revises the generation of API from the JSON spec:
  • Generates the elasticsearch/_async/client/... code using normal workflow
  • Uses unasync to translate async code to sync code in elasticsearch/client/...
• On Python 3.6+ import the AsyncElasticsearch, AsyncTransport, and AIOHttpConnection into the elasticsearch namespace.
• Add async helpers on Python 3.6+
  • Use unasync to generate these?
• Async unit tests
  • Move to pytest
  • Might be able to use unasync here too (nope)
  • On CI use a random gen seed, maybe on date, that "stripes" the test suite on Jenkins between all the connection classes
• Trim down the AsyncTransport class, doesn't need to override much
• AsyncTransport sniffing via async
  • May need to be different than the sync variant because most users will create AsyncElasticsearch in the global scope, outside an event loop.
  • Instead we can have sniff_on_start=True mean that sniffing happens in the background on the first perform_request() or call to __aenter__()
  • Can attempt to grab a currently running loop in constructor but if we don't find one fallback on the above method?
• Create documentation for AsyncElasticsearch and how to use
• Point to the new implementation on the elasticsearch-async repository
• Add aiohttp as an extra elasticsearch[async]?

[danielmitterdorfer]

Great to see asyncio support in the main code base! I left a couple of comments and suggestions mostly in the implementation of the new connection class.

[sethmlarson]

Closed in favor of #1230, thanks for all the early review comments!