this update of _study2shard_map should have had the mutex

This could have been the cause of https://groups.google.com/forum/?fromgroups&hl=en#!topic/opentreeoflife-software/za6y_Gn6KzQ

This update of the study ID to shard map should have been inside a with...lock block.

I checked for regressions, and this seemed fine. But it is hard to test for an effect of this fix because the bugginess is lack of thread safety (and there is a good chance that rebooting would fix the problem for a while even if this is not the only bug).