Threading interface for optimization/VI

[rok-cesnovar]

Should be done similar to how we handle it for sampling after #185 is merged.

[jgabry]

Someone just asked about this on the forum so following up here with the link so we remember to update the forum thread once this is done:

https://discourse.mc-stan.org/t/can-you-specify-multiple-threads-with-optimize-reduce-sum/18773