Use ai3/go-common for TLS client code and HA transport

proxy/proxy.go

proxy/transport.go

vendor/git.autistici.org/ai3/go-common/clientutil/retry.go

vendor/git.autistici.org/ai3/go-common/clientutil/tls.go

...r/git.autistici.org/ai3/go-common/clientutil/transport.go

vendor/git.autistici.org/ai3/go-common/misc.go

vendor/github.com/cenkalti/backoff/LICENSE

vendor/github.com/cenkalti/backoff/README.md

vendor/github.com/cenkalti/backoff/backoff.go

vendor/github.com/cenkalti/backoff/context.go

vendor/github.com/cenkalti/backoff/exponential.go

vendor/github.com/cenkalti/backoff/retry.go

vendor/github.com/cenkalti/backoff/ticker.go

vendor/github.com/cenkalti/backoff/tries.go

vendor/golang.org/x/net/LICENSE

vendor/golang.org/x/net/PATENTS

vendor/golang.org/x/net/context/context.go

vendor/golang.org/x/net/context/go17.go

vendor/golang.org/x/net/context/go19.go

vendor/golang.org/x/net/context/pre_go17.go