Feature/ipc

use a unix socket instead of a temportay file to signal activity
This change replace the use of a temportary file by a socket pair to
notify the master that the worker is still alive.

before we were doing the following:
 - create a tmp file / worker
 - write to it
 - check the last update bey using os.stat

now:

 - open a socket pair
 - write to it
 - on socket read we update the worker state

Possible improvement:

- instead of using SOCK_STREAM we could use SOCK_DGRAM to speed the
  exchanges since we don't need the reliability
- instead of using a select, depending on the platform we could use
  EPOLL, KQUEUE to speed the event handling in the arbiter.