Add zerostate proof

[Rexagon]

When a node boots from a genesis it needs a full state. However, when it boots from the latest persistent, zerostate is used only to check that the subsequent key block is valid. There is no need to store it fully.

I propose to introduce a "state proof" which must contain only a small subset of info required for ProofChecker. And all nodes in a hard-forked network will download it instead of the full state to speedup cold boot and reduce storage usage.

• Store zerostate as proof and use if for all checks instead of the full state. (Ability to create generic forked state and run network using prepared state #975)
• Optionally download only a zerostate proof on cold boot. (Add ability to sync node using only zerostate proof #1002)