Improve task cancellation management in SubscriptionManager

The widespread usage of the SubscriptionManager caused new crashes to
occur when swapping apps.

This was caused due to an access to Ndb memory after Ndb has been closed
from the app background signal.

The issue was fixed with improved task management logic and ensuring all
subscription tasks are finished before closing Ndb.

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

nostrdb/Ndb.swift

@@ -698,9 +698,13 @@ class Ndb {
         // Fetch initial results
         guard let txn = NdbTxn(ndb: self) else { throw .cannotOpenTransaction }
         
+        do { try Task.checkCancellation() } catch { throw .cancelled }
+        
         // Use our safe wrapper instead of direct C function call
         let noteIds = try query(with: txn, filters: filters, maxResults: maxSimultaneousResults)
         
+        do { try Task.checkCancellation() } catch { throw .cancelled }
+        
         // Create a subscription for new events
         let newEventsStream = ndbSubscribe(filters: filters)
         
@@ -717,6 +721,7 @@ class Ndb {
             // Create a task to forward events from the subscription stream
             let forwardingTask = Task {
                 for await item in newEventsStream {
+                    try Task.checkCancellation()
                     continuation.yield(item)
                 }
                 continuation.finish()
@@ -876,6 +881,7 @@ extension Ndb {
         case cannotConvertFilter(any Error)
         case initialQueryFailed
         case timeout
+        case cancelled
     }
     
     /// An error that may happen when looking something up