Consider how `LIMIT` should apply to `UNION` queries

[viktorvsk]

Currently if RELAY_CONFIG.max_limit is set to 100 and REQ command is sent by a client with 5 different filters (i.e. ["REQ", "subscription", {...}, {...}, {...}, {...}, {...}] the response in theory will contain 500 records maximum (100 per filter). Also, in case filters have #authors filter, NIP-26 delegation will trigger another UNION to each subquery.

We could limit resulting set, but it could be not clear for clients, so the best and the most obvious solution should be defined and enforced.