rust: update tonic dep to tokio 0.3 (#52)

This branch updates the `tonic` dependency to a Git dep on a branch that
uses Tokio 0.3 and the latest versions of other dependencies, such as
`bytes` and `tower`. This is necessary for linkerd/linkerd2-proxy#732.

Hopefully a new `tonic` release will be out soon and we can change the
git dependency back to a crates.io dependency...

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

.github/workflows/ci.yml

@@ -9,7 +9,7 @@ jobs:
     name: Rust build
     runs-on: ubuntu-18.04
     container:
-      image: docker://rust:1.41.0-buster
+      image: docker://rust:1.47.0-buster
     steps:
     # actions/checkout@v2
     - uses: actions/checkout@722adc6