Generalize ChunkInfo

[edsko]

IntersectMBO/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).

[edsko]

If we do this after the fact, we'll have to think about migrating existing on-disk chunk files that don't correspond to the chunk sizes we were expecting.

[edsko]

I'm envisioning something like

data ChunkInfo =
    UniformChunkSize ChunkSize
  | PostEBBs 
      ChunkSize   -- Pre-OBFT chunk size (with EBBs)
      SlotNo      -- Changeover point
      ChunkSize   -- Post-OBFT (without EBBs)

(which means translations are still fast, no need for something akin to CumulEpochSizes).

[edsko]

If we do this, then #704 could become more important.

[mrBliss]

First do #386, which will require an ImmutableDB migration anyway.