Apply stack deduplication heuristics from BHR frontend to output of bhr_collection.py

[squarewave]

We should apply some of the heuristics of the frontend, notably ignoring nested event loops, ignoring non-mozilla code (except the entry point to it), and ignoring SpiderMonkey internals.

This should dramatically reduce the output size, making the frontend load faster, and preparing us to output daily timeseries data for top hangs.

[skylarkning]

Will start working on this.

[skylarkning]

I have moved the front-end heuristics to back-end and have verified the consistency of the results (Left: Heuristics in Front-end; Right: Heuristics in Back-end). I am holding off for a PR since I am thinking of starting the migration of bhr_collection.py to mozilla-central and I am wondering if I should apply the patch after the migration.