Multiple filter requests may not be supported by all relays

Check if `max_filters` in NIP-11 is implemented around. Do most relays support multiple filters or not? How to tell?

If they only support one as probably [Khatru](https://github.com/fiatjaf/khatru) relays, then we can't use multiple filters (merging)

https://github.com/nostr-protocol/nips/pull/1821

Also see: https://github.com/nostr-protocol/nips/pull/1645