increase IDS_PER_CHUNK before going rc

[ThomasWaldmann]

IDS_PER_CHUNK is currently at 3, making sure that testers frequently get a item_ptrs list with more than a single entry - like people with really big archives would get later in production.

But before going to release candidates, we want to fix this to:

IDS_PER_CHUNK = MAX_DATA_SIZE // 40

That's about 500.000 and will result in chunks that are definitely below the max chunk size.

For most people, this means only a single entry in item_ptrs.