oc: add talloc reference to indexing context

We have a cache with user contexts (objc) which have mapping contexts (objc)
where we store the indexing context (c) which will be freed by openchange
code once the connection is closed.

Right now all indexing context created for an user are the same,
independently when in time was created or by which connection, etc...
So this is a valid workaround, but this has to be refactored to used the
indexing context of the current connection that is performing the
call to SOGo backend.

OpenChange/MAPIStoreMapping.m

@@ -114,6 +114,9 @@ MAPIStoreMappingKeyFromId (uint64_t idNbr)
     {
       ASSIGN (username, newUsername);
       indexing = newIndexing;
+      /* Workaround so all indexing context are valid and won't be freed. */
+      // TODO refactor indexing interface
+      talloc_reference(memCtx, indexing);
     }
 
   return self;