Modify NostrNetworkManager pipeline architecture

Previously, we combined the ndb and network stream within a "session
subscription" stream, which was teared down and rebuilt every time the
app went into the background and back to the foreground (This was done to
prevent crashes related to access to Ndb memory when Ndb is closed).

However, this caused complications and instability on the network
stream, leading to timeline staleness.

To address this, the pipeline was modified to merge the ndb and network
streams further upstream, on the multi-session stage, allowing the
session subscription streams to be completely split between Ndb and the
network.

For the ndb stream, we still tear it down and bring it up along the app
foreground state, to prevent memory crashes. However, the network stream
is kept intact between sessions, since RelayPool will now automatically
handle resubscription on websocket reconnection. This prevents
complexity and potential race conditions that could lead to timeline
staleness.

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

damus/Core/Nostr/RelayPool.swift

@@ -444,6 +444,7 @@ class RelayPool {
                 continue
             }
             
+            Log.debug("%s: Sending resubscribe request to %s", for: .networking, handler.sub_id, relayId.absoluteString)
             send(.subscribe(.init(filters: filters, sub_id: handler.sub_id)), to: [relayId], skip_ephemeral: shouldSkipEphemeralRelays)
         }
     }