Restructured the code in the following ways:

* add terminal states SUCCESS and FAILED
* split SOCK4 and SOCK5 states to be more clear
* use `bufq` for send/recv of SOCK messages
* reduce SOCKS4 states, more speaking names
* for most states, move code into static function
* reduce SOCKS5 states, more speaking names
* add helpers for traversing to FAILED state
* add helper to flush bufq
* add hepler to read minimum amount into bufq