Sets a summary sender to enable the manager to send sessions. If the sender is null, the sessions are not sent. If the sender is set, the manager will periodically search and send pending sessions, usually the one left over from processes that died unexpectedly. As a rule, the principal process is responsible to send the left over summary sessions and a subsidiary process only sends the current sessions summary if it is the last process of the group to exit. The manager deletes the session data while sending.