feat(tonic): Use BoxBody from http-body crate (#622)

As of `http-body` 0.4.1 its has had a `BoxBody` type similar to
`tonic::body::BoxBody`. It also has `Empty` and `Body::map_{data,err}`.

That means all the custom body things we had in tonic can basically be
replaced with that.

Note that this is a breaking change so we should merge this next time we
decide to ship a breaking release.

The breaking changes are:

- `tonic::body::Body` has been removed. I think its fine for users to
depend directly on `http-body` if they need this trait.
- `tonic::body::BoxBody` is now just a type alias for
`http_body::combinators::BoxBody<Bytes, Status>`. So the methods it
previously had are gone. The replacements are
  - `tonic::body::Body::new` -> `http_body::Body::boxed`
  - `tonic::body::Body::map_from` -> `http_body::Body::map_data` and
  `http_body::Body::map_err` depending on which part you want to map.
  - `tonic::body::Body::empty` -> `http_body::Empty`

Additionally a `Sync` bound has been added to a few methods. I actually
don't think this is a breaking change because the old
`tonic::body::Body` trait had `Sync` as a supertrait meaning the `Sync`
requirement was already there.

Fixes https://github.com/hyperium/tonic/issues/557

examples/Cargo.toml

@@ -173,7 +173,7 @@ prost-types = "0.7"
 hyper = "0.14"
 warp = "0.3"
 http = "0.2"
-http-body = "0.4"
+http-body = "0.4.2"
 pin-project = "1.0"
 # Health example
 tonic-health = { path = "../tonic-health" }