Add freelist cache.

This commit adds a cache to the freelist which combines the available free pages and pending free pages in a single map. This was added to improve performance where freelist.isFree() was consuming 70% of CPU time for large freelists.

/cc @mkobetic @snormore
@camilo It does some tricky to build the cache with only the pending pages. You're right though. It's kind of shitty. I'll change it.