Blender Git Loki

Fix T38000: vertex parent crash due to threading issues

Issue is causes by vertex parent modifies original BMesh from
a multiple threads. Ideally this is to be done as a separate
update callback for mesh datablock, but it's not so much simple
now (would need to do some re-arranges to DAG which might conflict
with the work from Ali or will double amount of work we did).

So for now use simple solution with mutex lock.

Based on the patch from Campbell Barton with some fixes to make
changes really thread-safe.

Differential Revision: https://developer.blender.org/D168