OpenBLAS

Fix thread races and infinite looping on systems with many cpus 

On systems with more than 64 cpus, blas_quickdivide will sometimes return zero which creates bogus workloads when used for the stride calculation. This then leads to threads spinning incessantly waiting for a status change that never happens, as seen in #1497.
This patch also fixes several data races that were found by helgrind and/or tsan while debugging the issue.