MB-8846 Use std::set instead of unordered::set.

The current mingw build env doesn't support unordered::set.

Change-Id: Ib4c6e635015ff464442430e138169d1fb067b063
Reviewed-on: http://review.couchbase.org/28257
Tested-by: Bin Cui <bin.cui@gmail.com>
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>