Possibility of deadlock, when there is other work in the threadpool

[ondrasouk]

In the connectivity::‎DisjointSet::flatten, there is std:sync::Barrier useage in a Rayon threadpool.

This is not good, since it can deadlock the whole process.

This actually is a problem since it causes flaky tests (tests by-default runs in parallel).

For now the workaround is to use separate threadpool.