Move AtomicUnorderedInsertMap to folly.

Summary: AtomicUnorderedInsertMap is a concurrent hash table that firmly
at the performance end of the generality <-> performance spectrum.
If you don't need updates (or can use your own concurrency control when
overwriting values), you never need to delete, and you can predict your
capacity perfectly, then you will get wait-free reads, lock-free inserts,
safe concurrent iteration, and excellent cache and performance outlier
behavior.  Arbitrary key and value types are supported.

Reviewed By: @yfeldblum

Differential Revision: D2145281