…and a least-connection load balancer.

Yet to be implemented: mTLS authn/z and RateLimiting

- Comments and consolidation of Config
- Simplify connection handling in proxy with less channels + goroutines

…ancer implementation.

…setting a sleep in the echo server,

and confirmed the tests failed, logging the i/o timeout. It is possible this doesn't work exactly how I think,
in that long-lived connections that don't send some type of heartbeat might expire. Will test that once I have
the server component.

```
❯ make test
go test ./...
2024/02/28 21:27:59 INFO proxy ready listening=127.0.0.1:61459 targets=127.0.0.1:61458
2024/02/28 21:28:01 ERROR idle timeout exceeded error="readfrom tcp 127.0.0.1:61459->127.0.0.1:61460: read tcp 127.0.0.1:61461->127.0.0.1:61458: i/o timeout"
2024/02/28 21:28:01 ERROR copying data error="readfrom tcp 127.0.0.1:61459->127.0.0.1:61460: read tcp 127.0.0.1:61461->127.0.0.1:61458: i/o timeout"
2024/02/28 21:28:01 ERROR idle timeout exceeded error="readfrom tcp 127.0.0.1:61461->127.0.0.1:61458: read tcp 127.0.0.1:61459->127.0.0.1:61460: i/o timeout"
2024/02/28 21:28:01 ERROR copying data error="readfrom tcp 127.0.0.1:61461->127.0.0.1:61458: read tcp 127.0.0.1:61459->127.0.0.1:61460: i/o timeout"
```

… on connection closing to prevent

a goroutine from getting stuck. Simplify the public API.

* Implement a Token Bucket rate limiter

* Misc cleanup, comments

* RateLimitManager gracefully handles individual RateLimiters being closed already.

…initial-library