Further improvements to app lifecycle handling

- Resend subscription requests to relays when websocket connection is
  re-established
- More safeguard checks on whether Ndb is opened before accessing its
  memory
- Cancel queued unsubscribe requests on app backgrounding to avoid race
  conditions with subscribe requests when app enters the foreground
- Call Ndb re-open when Damus is active (not only on active notify), as
  experimentally there have been instances where active notify code has
  not been run. The operation is idempotent, so there should be no risk
  of it being called twice.

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

nostrdb/Ndb.swift

@@ -701,9 +701,10 @@ class Ndb {
                 terminationStarted = true
                 Log.debug("ndb_wait: stream: Terminated early", for: .ndb)
                 streaming = false
-                ndb_unsubscribe(self.ndb.ndb, subid)
                 Task { await self.unsetCallback(subscriptionId: subid) }
                 filtersPointer.deallocate()
+                guard !self.is_closed else { return }   // Double-check Ndb is open before sending unsubscribe
+                ndb_unsubscribe(self.ndb.ndb, subid)
             }
         }
     }