Update tower-grpc dependencies, switch to tower-hyper

[mzabaluev]

All dependencies of tower-grpc in its git master branch have graduated to be versioned on crates.io. It can be used with a crates.io version of tower-hyper as the HTTP stack. This means we can achieve build stability with revision-locked tower-grpc, but the HTTP stack crate needs to be switched from tower-h2 to tower-hyper.

Things to do

Blockers in tower-hyper

• Introduce client::Background (Introduce client::Background tower-rs/tower-hyper#46)
• Make client::ConnectFuture public (Make client::ConnectFuture public tower-rs/tower-hyper#48)
• Make client::ResponseFuture public (Make client::ResponseFuture public tower-rs/tower-hyper#47)
• Wait for a crates.io release incorporating the above changes.

network-grpc

• Finalize the porting branch after tower-hyper issues have been resoved and submit a PR.

[mzabaluev]

The switch to tower-hyper is complete. The dependency still has to be locked to a git revision because the necessary changes did not yet make their way into a release, but it does not cause any instability any more.