Minimize disk dependency for chunk validators

[walnut-the-cat]

Currently chunk validators need at least Blocks. But it should be possible to reduce disk dependency to (usually) short range of BlockHeaders. The range may be non-trivial because state witness can cover arbitrary range of blocks in case of missing chunks.

It's not a straightforward change, because during implementation we assumed dependency on Chain and EpochManager everywhere. And I think BlockSync is always enabled by default so we need an option to tweak that. We could even put BlockHeaders to memory but that's hard implementation wise, and makes chunk validator less reliable - if stopped, it'd need to download headers from somewhere before being able to perform validation again.

But if one implements that, disk usage will be reduced to, usually, near zero. Doesn't look like a high priority feature in the protocol, but nice and meaningful to have.

relevant discussion: link