MEDIUM: threads: Adds a set of functions to handle sync-point

MEDIUM: threads: Adds a set of functions to handle sync-point

A sync-point is a protected area where you have the warranty that no concurrency
access is possible. It is implementated as a thread barrier to enter in the
sync-point and another one to exit from it. Inside the sync-point, all threads
that must do some syncrhonous processing will be called one after the other
while all other threads will wait. All threads will then exit from the
sync-point at the same time.

A sync-point will be evaluated only when necessary because it is a costly
operation. To limit the waiting time of each threads, we must have a mechanism
to wakeup all threads. This is done with a pipe shared by all threads. By
writting in this pipe, we will interrupt all threads blocked on a poller. The
pipe is then flushed before exiting from the sync-point.