Use asyncio instead of threading

[vickumar1981]

The current Future implementation uses python's threading library, to maintain backwards compatibility with Python 2.

If Python 2 is no longer supported, then the class can instead use asyncio.

[vickumar1981]

As of 1.0.4, requires Python >= 3.6.

[vickumar1981]

Open question: How to pass event loop into future class? Should a new class be created for using asyncio, an AsyncFuture?