MEDIUM: mworker: replace the master pipe by socketpairs

MEDIUM: mworker: replace the master pipe by socketpairs

In order to communicate with the workers, the master pipe has been
replaced by a socketpair() per worker.

The goal is to use these sockets as stats sockets and be able to access
them from the master.

When reloading, the master serialize the information of the workers and
put them in a environment variable. Once the master has been reexecuted
it unserialize that information and it is capable of closing the FDs of
the leaving children.