Commit to "prev_root" in block headers

[antiochp]

Related - #1555 (FlyClient).
This is definitely not a full flyclient impl but it moves us a step in that direction.

Starting this issue to track concrete steps toward committing to prev_root in our block headers.

Tasks -

• implement header MMR (see The Header MMR (One MMR To Rule Them All) #1716)
  • merge onto master branch so we can test this early (not consensus breaking)
  • if happy with impl we can merge onto testnet4 branch
• on testnet4 branch introduce prev_root to block headers (leave previous hash in place)
  • testnet4 PR [T4] Commit to prev_root in block headers #1764
  • ideally this happens prior to testnet4 release (so we can avoid a hardfork to introduce it)

• subsequently on testnet4 (or later) hardfork to remove previous hash on headers

I think -
a) We can get this into testnet4 without too much hassle.
b) And that the benefits to doing so now are worth it.

@ignopeverell @yeastplume @tromp does this approach make sense?

[tromp]

That makes perfect sense. Before removing prevhash, have as many sanity checks as possible that it's consistent with the header MMR.

[yeastplume]

Yes, seems like a good approach, we can try it and work on removing previous hash if all goes well.

[antiochp]

Closing this. We are up and running on pre-testnet4. 🚀