Share log file descriptor between cluster master and workers

Using IPC messaging for sending log lines as multiple issues:

1/ it impacts node process performances: http://www.subbu.org/blog/2012/03/node-js-cost-of-ipc & http://aphyr.com/posts/244-context-switches-and-serialization-in-node
2/ when a worker is disconnected from the master (ex: when using https://github.com/isaacs/cluster-master), workers cannot log anymore

This PR use file descriptor sharing (like it would have been done with any other fork friendly language).