Race condition in Tinkergraph index creation

My colleage Fabian Yamaguchi <fabs@shiftleft.io> discovered a race condition in tinkergraph's index creation. He fixed it by simply replacing `parallelStream` with `stream`. Quoting his analysis:

> So, reading the code, you see that this.put is called in parallel, but that method seems to contain a race as get is called on the index, checked for null, and a subsequent write is performed. It still seems like using stream here fixes the problem we've been seeing, and the performance hit is not significant.