This is the software combining tree barrier from the MCS paper. Currently,
it uses a binary tree; it may be changed later to use an n-ary tree.
Validation (combining_validation.c in regressions/ck_barrier/validate)
has also been added.
Instead of executing a join on the threads, aggregate existing state
of their counters. This can mean one missed increment per thread,
which is not a big deal.
Samy Al Bahra
David Joseph
Samy Al Bahra