Multi-session subscriptions and RelayPool reopening

This commit implements nostr network subscriptions that survive between
sessions, as well as improved handling of RelayPool opening/closing with
respect to the app lifecycle.

This prevents stale data after users swap out and back into Damus.

Signed-off-by: Daniel D’Aquino <daniel@daquino.me>

nostrdb/Ndb.swift

@@ -712,6 +712,7 @@ class Ndb {
         return AsyncStream<StreamItem> { continuation in
             // Stream all results already present in the database
             for noteId in noteIds {
+                if Task.isCancelled { return }
                 continuation.yield(.event(noteId))
             }