undo a (error behavior correction) fix for get_num_threads which is already in develop, but not affecting this hotfix

This reverts commit 2442c067.

Feature/issue 660 add periodic cov

[Jenkins] pull fn out to library, remove cruft

Vectorized a bunch of the discrete rngs

More granular test stages for better replay on Jenkins

Feature/integrate 1d boost

Merge branch 'feature/integrate-1d-boost' of https://github.com/stan-dev/math into feature/integrate-1d-boost

Try to drop libc++ for clang

this fails for a couple of commented out tests