Add a lock on a file to prevent two processes running at the same time.

Take a lock on a file, fail if another process is already locking.

Prevents two jobs of `brigade` to overlaps (if a cron job is not finished yet). This can't occur right now because the http monitoring handler anecdotally served as a hacky lock by binding a socket =3.

r: @camilo
is int(file.Fd()) warrantied to return the right fd number ?