SpaceManager keeps separate lists for the different chunk sizes. This is unnecessary.

It needs to keep a list of in-use chunks in order to free them all at destruction time. But there is no need to keep them separated by chunk size, and it would make coding easier if it wouldn't.

Note that we still need to be able to know how many chunks for a given chunk size are in use by the SpaceManager - see e.g. calc_chunk_sizes(). However, that is better done with counters instead of summing up the chunks by iterating over per-size-kept lists.