nip01: add `until` to request filter

fixes https://github.com/fiatjaf/nostr/issues/39.

nips/01.md

@@ -66,6 +66,7 @@ Clients can send 3 types of messages, which must be JSON arrays, according to th
   "#e": <an event id that is referenced in an "e" tag>,
   "#p": <a pubkey that is referenced in a "p" tag>,
   "since": <a timestamp, events must be newer than this to pass>,
+  "until": <a timestamp, events must be older than this to pass>,
   "authors": <a list of pubkeys, the pubkey of an event must be one of these>
 }
 ```