oc: cached usercontext updates indexing_context

Instead of use always the one given on initialization

OpenChange/MAPIStoreUserContext.h

@@ -68,6 +68,8 @@
 - (id) initWithUsername: (NSString *) newUsername
          andTDBIndexing: (struct indexing_context *) indexing;
 
+- (void) updateIndexing: (struct indexing_context *) indexing;
+
 - (NSString *) username;
 - (SOGoUser *) sogoUser;