Lock-free parallel disjoint set data structure

This is a small self-contained C++11 implementation of the UNION-FIND data structure with path compression and union by rank and a few extras It supports concurrent find(), same() and unite() calls as described in the paper

Wait-free Parallel Algorithms for the Union-Find Problem by Richard J. Anderson and Heather Woll

In addition, this class supports optimistic locking (try_lock()/unlock()) of disjoint sets and a combined unite+unlock operation for pairs of sets.