switch the IndexType and Allocator in AtomicUnorderedInsertMap template

Summary: Our use case need to have the IndexType Configurable, but in this diff D2583752, lint error shows
  Code from folly::detail is logically private, please avoid use outside of folly.
In order to fix this lint error, I switch the IndexType and Allocator in the AtomicUnorderedInsertMap template.

Reviewed By: nbronson

Differential Revision: D2610921

fb-gh-sync-id: ae81b41e7c8c971f26c61b8c67dabeadff379584