oc: cached usercontext updates indexing_context

Instead of use always the one given on initialization

OpenChange/MAPIStoreMapping.m

@@ -119,6 +119,11 @@ MAPIStoreMappingKeyFromId (uint64_t idNbr)
   return self;
 }
 
+- (void) updateIndexing: (struct indexing_context *) newIndexing
+{
+    indexing = newIndexing;
+}
+
 - (void) dealloc
 {
   [username release];