Restrictions on R+W, I+O

[mikedilger]

Essentially we should have gossip test relays and either restrict or warn about their suitability for usage:

nip11.limitation.min_pow_difficulty -- Warn for OUTBOX, Warn for INBOX nip11.limitation.payment_required, or nip11.fees, or nip11.payments_url -- Warn for OUTBOX, Error for INBOX nip11.limitation.restricted_writes -- Warn for OUTBOX, Warn for INBOX