Add support for async I/O

[sethmlarson]

Depends on: elastic/elasticsearch-py#1203

Should be a fairly similar approach to above.

• Add an [async] extra which will install elasticsearch[async].
• Add AsyncSearch, AsyncMultiSearch, and AsyncUpdateByQuery classes

[zikphil]

@sethlarson Any timeframe for implementation?

[sethmlarson]

@zikphil I don't have a timeframe available currently, my guess is in the next few months.

[dnutiu]

Would be great to have this

[monkeyDG]

Agreed, this is something I'm looking forward to

[bouraine]

Yeah It would be great. Looking forward to use this in my projects.

[631068264]

When finish?

[erichaus]

elastic/elasticsearch-py#1230 looks like it's all merged?

[sethmlarson]

Update: Still haven't found time to work on this feature, it is on my to-do list. As always, community PRs are welcome if you're willing to take a stab at this large change.