Support plugable Timer implementation

[xofyarg]

Feature Request

Crates

tonic

Motivation

Tonic's builtin transport rely on tokio runtime, but with some simple tweaking, I managed to make it work with another runtime, by providing a customized hyper::rt::Executor and Connector(tower_service::Service), ... except for the timer, which is buried deeply in the channel implementation.

Proposal

Because the callsite accepts a hyper::rt::Timer naturally, I wonder if there's any chance to expose it somehow via an interface, to allow the users set it to implementations other than tokio.

P.S.

I split the feature channel into channel and channel-full, and feature guarded some of the code related to tokio. So when only channel is enabled, it's possible to use most of the builtin channel without rely on tokio(#152). Please let me know if this is something you wanted, then I can start a PR.

channel = [
  "dep:hyper", "hyper?/client",
  "dep:hyper-util", "hyper-util?/client",
  "dep:tower", "tower?/buffer", "tower?/discover", "tower?/util",
  "dep:tokio",
]
channel-full = [
  "channel",
  "hyper-util?/client-legacy", "hyper-util?/tokio",
  "tower?/balance", "tower?/limit",
  "tokio?/time",
  "dep:hyper-timeout",
]