proxy: v2.85.0 (#4010)

This release fixes a bug in the proxy's logging subsystem that could
cause the proxy to consume memory until the process is OOMKilled,
especially when the proxy was configured to log diagnostic information.

The proxy also now properly emits `grpc-status` headers when signaling
proxy errors to gRPC clients.

This release upgrades the proxy's Rust version, the `http` crate
dependency to address RUSTSEC-2019-0033 and RUSTSEC-2019-0034, and the
`prost` crate dependency has been patched to address RUSTSEC-2020-02.

---

* internal: Introduce a locking middleware (linkerd/linkerd2-proxy#408)
* Update to Rust 1.40 with new Cargo.lock format (linkerd/linkerd2-proxy#410)
* Update http to v0.1.21 (linkerd/linkerd2-proxy#412)
* internal: Split retry, http-classify, and http-metrics (linkerd/linkerd2-proxy#409)
* Actually update http to v0.1.21 (linkerd/linkerd2-proxy#413)
* patch `prost` 0.5 to pick up security fix (linkerd/linkerd2-proxy#414)
* metrics: Make Counter & Gauge atomic (linkerd/linkerd2-proxy#415)
* Set grpc-status headers on dispatch errors (linkerd/linkerd2-proxy#416)
* trace: update `tracing-subscriber` to 0.2.0-alpha.4 (linkerd/linkerd2-proxy#418)
* discover: Warn on discovery error (linkerd/linkerd2-proxy#422)
* router: Avoid large up-front allocations (linkerd/linkerd2-proxy#421)
* errors: Set correct HTTP version on responses (linkerd/linkerd2-proxy#424)
* app: initialize tracing prior to parsing env vars (linkerd/linkerd2-proxy#425)
* trace: update tracing-subscriber to 0.2.0-alpha.6 (linkerd/linkerd2-proxy#423)