Make Immutable DB independent from EpochSize

[edsko]

At the moment, the immutable DB stores blocks per epoch. Unfortunately, this introduces a dependency between the immutable DB and the ledger state, because the ledger state tells us when epoch sizes change. Instead we should give the immutable DB a fixed "chunk" parameter, with a precondition that if there are EBBs present, they must line up with the first slot in each chunk. Unfortunately, this means that the size of the files stored on disk will go down a factor of 10 or even 20 (depending on the choice of the Praos f parameter), but after discussing this with @dcoutts , we considered this an acceptable compromise.

This is blocking the work on the hard fork combinator; indeed, it is blocking the work at #1698 on #1637 / #1205 .

[edsko]

WIP PR at #1717.

[edsko]

Closed in #1750.