Use net module instead of unix-dgram for unix protocol

  - core `net` module does support unix sockets so there is no need for
    `unix-dgram` which also requires the compilation of binding and we are
    better to keep it as pure JS solution
  - give a name to anonymous functions to improve stack traces
  - improve callbacks handling by calling them using `process.nextTick`
    instead of doing it right away which breaks asynchronous nature of
    the code
  - increase `inFlight` counter for all protocols as it is being decreased
    for all protocols as well
  - add missing semicolons
  - add whitespaces around operators for consistency