Reenable the advesarial FSA based page cache test

Muninn was already passing it and we don't have the StandardPageCache any more
Simplify the PageCursor pooling

Since we have MuninnPageCursorPools for every PagedFile, and since a thread can
only use a single cursor at a time, per PageCache, without risking deadlocks,
it makes no sense to pool more than a single cursor.

Therefore, this commit drops the freelist concept, so we just have a single
thread-local variable that points to whatever cursor instance has been created.