lib-index: mail_index_sync_map() - Don't try to-resync extension updates This was done to call extension record sync handlers, but the previous commit removes them. Fixes a problem where obsolete cache offsets were used in some situations: - Some cache updates are from external transactions and some are from non-external transactions. This is because cache offset updates are being added by whatever the parent index transaction is. - When mail_index_sync_map() is mapping MAIL_INDEX_SYNC_HANDLER_FILE, it has already synced the map. But it's calling mail_index_sync_record() for non-external transactions to call expunge handlers and extension update handlers. It's calling the regular mail_index_sync_record() to do this work. - But mail_index_sync_record() is actually still updating the map. So now mail_index_sync_record() is called for all non-external cache updates, but not for external cache updates! And since these are somewhat randomly either external or non-external, the end result is that the cache offset may be obsolete.