src: add 'unix-connect' protocol

- Starting with unix-dgram@0.1.0 there is support for using `connect` in unix
  DGRAM sockets. It allows to have some kind of congestion control via the
  `congestion` and `writable` events.
- This patch adds the `unix-connect` protocol that uses this functionality. While
  in congestion it enquees the messages. Once in writable mode it flushes the
  queue and keeps sending messages as normal.
- It avoids problems like the ones described in https://github.com/bnoordhuis/node-unix-dgram/issues/4.