Partition Tor Browser heap with jemalloc

We should investigate performance and robustness of a partitioned heap using jemalloc4. Some links to get started:

http://ggp.name/2014/10/15/Partitioned-heap-in-Firefox-pt1.html http://ggp.name/2015/03/03/Partitioned-heap-in-Firefox-pt2.html

And tjr's summary:

https://lists.torproject.org/pipermail/tbb-dev/2015-October/000309.html

One thing worth considering is whether it should be random partitions or not.