[scudo] Add PageReleaseContext to convey page usage status.

PageReleaseContext contains all the information needed for determing if
a page can be released. Splitting out the context increases the flexibility
of heterogenous free lists in the future. Also rename PackedCounterArray to
PageMap.

Reviewed By: cryptoad, cferris

Differential Revision: https://reviews.llvm.org/D133895