Deprioritize extents that are used for Huge allocations.

[deadalnix]

These extent may end up fragmenting the heap when the huge allocation is freed, but other allocation remain on the block. If another huge allocation of the same size were to be made, it wouldn't be possible to put it in the same spot.

For this reason, we ought to make blocks that are (or were) part of huge allocations the lowest priority possible.