yamux writes logs directly to stderr

The RPCClient uses the default yamux client configuration settings.

https://github.com/hashicorp/go-plugin/blob/0c19a1387af146eaa3335dad0a021741344ac3e9/rpc_client.go#L63

This means that even though we passed a logger to the plugin client, the yamux logs ignore it and are printed directly to stderr. It is causing us a lot of confusion when reading the plugin logs, since any problems with yamux aren't showing up where we expect to find them.

As an aside, if anyone has information on why yamux sometimes logs stuff like the following, I'd appreciate any pointers. I can't tell if this is expected or it indicates that I'm using the plugin framework incorrectly.

```
2021/03/16 23:13:31 [ERR] yamux: Failed to write header: write unix @->/tmp/plugin383370083: write: broken pipe
2021/03/16 23:15:41 [ERR] yamux: Failed to write header: write unix @->/tmp/plugin806433500: use of closed network connection
```