Optimise back-endorsement chain handling

[maxtropets]

We may want to optimise back-endorsement chain prefetch.

In #7711 we went the simpler way and only swapped receipt format, and didn't include the chain in the snapshot.

We may still do it though, here's the up/down sides of snapshot-backed-in-chain, either separately or bake it into the snapshot too.

Cons

• Snapshot size may grow?
  • Although we can make it optional of too big, or only fill it up to the certain size and fetch the rest

Pros

• Piggy-pack on snapshot auto-download

[maxtropets]

We've done an optimisation recently to not block the available receipts: #7922.

With this in mind, I doubt we need any optimisation for the foreseeable future, unless some projects start to have thousands of DRs as a part of the normal operations

Therefore, closing this ticket.