fix: wait for pending transactions before db close

This was triggered by some archive decryption test I was writing and
possibly also is triggered in production a few times. Waiting for
running transactions to complete before closing sounds sensible.

lib/src/database/indexeddb_box.dart

@@ -80,7 +80,8 @@ class BoxCollection with ZoneTransactionMixin {
 
   Future<void> close() async {
     assert(_txnCache == null, 'Database closed while in transaction!');
-    return _db.close();
+    // Note, zoneTransaction and txnCache are different kinds of transactions.
+    return zoneTransaction(() async => _db.close());
   }
 
   @Deprecated('use collection.deleteDatabase now')

lib/src/database/sqflite_box.dart

@@ -66,7 +66,7 @@ class BoxCollection with ZoneTransactionMixin {
         },
       );
 
-  Future<void> close() => _db.close();
+  Future<void> close() => zoneTransaction(() => _db.close());
 
   @Deprecated('use collection.deleteDatabase now')
   static Future<void> delete(String path, [dynamic factory]) =>