Splitting the atoms table (bug 1434598)

I'm working on improving parallel parsing performance by partitioning
the atoms table into multiple smaller tables, each with their own lock. 
This will remove the exclusive access lock and allow concurrent
allocation into the atoms zone (only requiring the GC lock on arena
allocation).

I'm advertising this to the list because this is a pretty large change.

There are more details here:
https://bugzilla.mozilla.org/show_bug.cgi?id=1434598#c12

Jon