templatize AtomicUnorderedInsertMap's internals to allow big maps

templatize AtomicUnorderedInsertMap's internals to allow big maps

Summary: AtomicUnorderedInsertMap used 32 bit index values internally.  2 bits
were stolen, limiting the capacity to 2^30.  This diff makes the internal
index type a template parameter, so you can make really big maps if you
want (at the expense of bigger map overhead).  The easiest way is to
substitute AtomicUnorderedInsertMap64.

Reviewed By: yfeldblum

Differential Revision: D2574338

fb-gh-sync-id: a74994b6da1046a149c2e7763c3dc19e35d9840b