consolidate which wallets are allowed to use Lightning

[SomberNight]

from comment ac1c9f0#commitcomment-151252517

• the matrix of supported wallet "types" is more complex than what the commit message suggests.
  • roughly the following wallets can have lightning enabled on 4.5.x (all with p2wpkh addresses):
    (1) electrum seeds, wallet created post-4.1, so lightning_xprv is deterministic
    (2) electrum seeds, wallet created pre-4.1, so lightning_xprv is non-deterministic
    (3) software bip39 seeds or raw zprvs. lightning_xprv is non-deterministic
    (4) watch-only/hw keystores. lightning_xprv is non-deterministic
  • notably the commit prevents enabling LN for type-3, but does not make any mention of this in the commit message.
• the motivation for the commit is probably to prevent opening anchor channels with type-4 wallets as those wallets cannot have hot privkeys for onchain UTXOs in RAM (which is needed to be able to do automatic actions sometimes needed for anchor channels, such as bringing our own UTXOs as inputs for HTLC txs)
  • (which would have been nice to say in the commit message :P)
• there is also the concern of how to handle existing wallet files with LN already enabled, which this commit does not address.
  • we might want to prevent opening anchor channels in some cases
    • in particular, probably at least for type-4?

[SomberNight]

@ecdsa and I discussed this today, and the conclusion was that for now we could keep the 4.5.x status-quo except:

• for type (4) wallets: disable opening new channels. and prevent freshly enabling LN.

[accumulator]

Maybe, for swiss-army-knife sake, still allow this from the CLI behind a --iknowwhatimdoing toggle?