x/pkgsite: prevent teeproxy from bringing pkg.go.dev down

[shaqque]

The teeproxy was implemented as a way to forward requests from godoc.org to pkg.go.dev and ensure that pkg.go.dev can handle the current workload of godoc.org.

We should make sure that abnormally large traffic from godoc.org does not bring down pkg.go.dev, e.g. by rate limiting and implementing the circuit breaker pattern.

[gopherbot]

Change https://golang.org/cl/238298 mentions this issue: internal/teeproxy: add rate limiting and circuit breaker