[BREAKING] fix: prevent spv attack by checking coinbase proof

[sander2]

• To-do: decide what we want to do with the bitcoin data. It's now only 2700 or so blocks, and already 60+ megabytes..
  • Decided to do only 2 txs per blocks. Now the dataset is about 11 mb for > 2*2016 blocks

Closes #1073

[gregdhill]

Most of my comments are non-blocking but requesting changes specifically only for adding documentation and cleaning up the unused code. All other comments are mostly nitpicks, but in general the solution looks good.

[gregdhill]

I also verified that we can indeed use the first txid returned by esplora to get the coinbase transaction.

Here's a quick one-liner to check that via curl (docs):

curl -s https://blockstream.info/api/tx/$(curl -s https://blockstream.info/api/block/$(curl -s https://blockstream.info/api/block-height/799160)/txids | jq -r '.[0]') | jq '.vin'