Lnd doesn't recreate admin macaroon unless invoice & readonly macaroons are deleted too

[sangaman]

Right now, if you delete only the admin.macaroon file and restart lnd, the admin.macaroon file won't be regenerated. WIthout it, things like lncli won't work. If you delete both readonly.macaroon and invoice.macaroon then lnd will regenerate all three.

I figure lnd should regenerate these independently, or at least the admin macaroon since it's an important part of using lnd (and without it, you can't bake macaroons).

Does this make sense? I wanted to get some input before working on a PR since this isn't really a bug.

[guggero]

Yeah, this original default behavior is quite confusing, especially as we added more subserver macaroons which aren't treated the same way as the three "default" macaroons. So I think it makes sense to unify the behavior and re-generate individual macaroons if they are missing.