Bug 898558 - Rework jemalloc_stats so it exposes how much memory is used for bookkeeping. r=glandium

This patch also gets rid of the redundant "committed" entry, so now
there's no confusion as to which entries overlap.

--HG--
extra : rebase_source : 429f3d44011f02dda43aa10b077c917c4d02fe50
Bug 898558 - Rework the jemalloc heap memory reporters. r=njn

Now we have the following hierarchy

  explicit/
    heap-overhead/
      bookkeeping
      waste
      page-cache (previosuly known as dirty)

In addition, heap-committed-unused-ratio is now known as heap-overhead-ratio.

--HG--
extra : rebase_source : 8d2ca75904dfb3c0dfd1ba9a132f828436b734c6