proxy: v2.125.0 (#5392)

This release features a change to the proxy's cache eviction strategy to
ensure that clients (and their load balancers) are reused by new
outbound connections. This can dramatically reduce memory consumption,
especially for busy HTTP/1.1 clients.

Also, the proxy's HTTP detection scheme has been made more robust.
Previously, the proxy would perform a only single read to determine
whether a TCP stream was HTTP, which could lead to false positives. Now,
the proxy reads until at least the first newline, which is what the HTTP
parser actually needs to make a proper determination. With this, the
default dispatch timeouts have been increased to 5s to accomodate
connection pools that may not issue an immediate request.

Furthermore, this release includes an upgrade to Tokio v0.3 and its
associated ecosystem.

---

* update buffers to use Tokio 0.3 MPSC channels (linkerd/linkerd2-proxy#759)
* Update the proxy to use Tokio 0.3  (linkerd/linkerd2-proxy#732)
* Rename DetectHttp to NewServeHttp (linkerd/linkerd2-proxy#760)
* http: more consistent names for body types (linkerd/linkerd2-proxy#761)
* io: simplify the `Io` trait (linkerd/linkerd2-proxy#762)
* trace: nicer traces in tests, clean up trace configuration (linkerd/linkerd2-proxy#766)
* Ensure that services are held as long they are being used (linkerd/linkerd2-proxy#767)
* outbound: add stack tests for http (linkerd/linkerd2-proxy#765)
* cache: Ensure that actively held services are not evicted (linkerd/linkerd2-proxy#768)
* cache: Only spawn a single task per cache entry (linkerd/linkerd2-proxy#770)
* test: make integration tests shut up (linkerd/linkerd2-proxy#771)
* metrics: Add support for microsecond counters (linkerd/linkerd2-proxy#772)
* Add a protocol label to stack metrics (linkerd/linkerd2-proxy#773)
* detect: Make protocol detection more robust (linkerd/linkerd2-proxy#744)