Port to Async/Await

[ghost]

@stepancheg as I have already mentioned in grpc-rust#164 I am wondering if it would be in the plan to port this repo to new async/await syntax together with futues 0.3.x

I am aware that tokio 0.2 is still in alpha, but I am happy to start porting small parts of the crate to the new syntax. What are your thoughts on this?

[stepancheg]

I probably don't have time to do it myself, but patches are welcome, and yes, async/await is the future.

[stepancheg]

tls-api crate upgraded to tokio 0.2. So now rust-http2 can be.

[stepancheg]

This crate upgraded to tokio 0.2.

So now:

• more tests needed
• convert more code to modern idioms (including async/await)
• convert grpc-rust to tokio 0.2

[stepancheg]

Done.