Generalize `ChunkInfo`

input-output-hk/ouroboros-network#1750 moved the immutable DB away from being epoch based to being chunk based. However, it only supports a simple uniform chunk size. Changing that should now be relatively easy; the hardest part will be to update the tests. It will be _useful_ to change this because if we use a single uniform chunk size, _and_ those chunks can contain EBBs (as they can for Byron), then that chunk size _must_ be set to the _Byron_ epoch size -- meaning that the chunks for Shelley will not contain very many blocks. The immutable DB can support multiple chunk sizes in principle, and moreover, those chunk sizes _only_ need to be related to epoch sizes for chunks that contain EBBs. Hence, we could (and proabably should) pick a different chunk size for the post-EBB era (Shelley, or indeed the OBFT hard fork).