lib/threads: intro of the threads lib, with Thread, Mutex and Barrier

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib/threads: optional redefs of collections to be thread-safe

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib/threads: intro new thread-safe concurrent collections

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib/threads: intro an example `concurrent_array_and_barrier`

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>