Split up allocator block lists

[jfrimmel]

Currently, the allocator has a single linked list of blocks. This is rather bad, since the two primary operations only consider a subset of those blocks:

1. allocation is only interested in free blocks, hence each allocated block is ignored
2. deallocation is only interested in used blocks, ignoring the free ones.

Therefore it makes sense to have two separate linked lists, one for free and one for used blocks. This should make maintaining the lists somewhat easier as well, as the lists have now a narrower scope. It certainly will speed up the searching for blocks, since only candidates for the operation in question are considered.