some cleanup of SocksToRtc logging

Because logging is important...:

* performance improvement: rather than constructing the entire message string, use placeholders with an array
* INFO level shows just session creation and destruction
* many more DEBUG messages, e.g. show which events caused the server or session to close

Example for fetching www.example.com in DEBUG:
```
SocksToRtc I [10/20 19:53:10.770] created new session c0
SocksToRtc D [10/20 19:53:10.779] opened datachannel for session c0
SocksToRtc D [10/20 19:53:10.805] session c0 connected to remote endpoint {"address":"2606:2800:220:6d:26bf:1447:1097:aa7","port":80}
SocksToRtc D [10/20 19:53:10.808] session c0 (TCP open): client socket received 79 bytes
SocksToRtc D [10/20 19:53:10.816] session c0 (TCP open): datachannel received 1591 bytes
SocksToRtc D [10/20 19:53:10.822] session c0 (TCP closed): client socket closed (REMOTELY_CLOSED)
SocksToRtc D [10/20 19:53:10.823] session c0 (TCP closed): freeing resources
SocksToRtc I [10/20 19:53:10.824] discarded session c0 (0 remaining)
SocksToRtc D [10/20 19:53:10.828] session c0 (TCP closed): datachannel closed
```

Do you agree it's an improvement?

Next pull request would be RtcToNet.
Style: might as well put this open bracket on the next line.