Most is taken from lightningd/bitcoind and adapted. This currently
exposes 5 commands:
- `getchaininfo`, currently called at startup to check the network and
whether we are on IBD.
- `getrawblockbyheight`, which basically does the `getblockhash` +
`getblock` trick.
- `getfeerate`
- `sendrawtransaction`
- `getutxout`, used to gather infos about an output and currently used by
`getfilteredblock` in `lightningd/bitcoind`.
