feat(transport): support customizing Channel's async executor

[davidpdrsn]

Motivation

At Embark we've been working on integrating tokio-console into our project and have used hyper::client::Builder::executor to pass our own executor that spawns named tasks. However its not possible to do that tonic, so tasks spawned by tonic are left unnamed.

Solution

• Add tonic's own Executor trait. Its identical to hyper's but added here not to have a public dependency on hyper.
• Added Endpoint::executor builder method to support setting another executor.
• Spawn tower::buffer::Buffer tasks on the given executor.
• Still uses tokio::spawn by default.

I intentionally didn't touch the server side because we don't need that at Embark but could do that if we think its relevant.

[LucioFranco]

Not opposed to this at all, though I am happy to just use hyper's since there is a plan for 1.0 there.

[davidpdrsn]

I've moved to hyper's Executor trait.