Add "batching" state to Bruce's message state tracking mechanism, and
show per-topic counts of batched messages in "queued message info" part
of Bruce's web interface.

	modified:   doc/status_monitoring.md
	modified:   src/bruce/msg.h
	modified:   src/bruce/msg_dispatch/broker_msg_queue.cc
	modified:   src/bruce/msg_dispatch/broker_msg_queue.h
	modified:   src/bruce/msg_state_tracker.cc
	modified:   src/bruce/msg_state_tracker.h
	modified:   src/bruce/router_thread.cc
	modified:   src/bruce/web_interface.cc
	modified:   src/bruce/web_request_handler.cc
	modified:   src/bruce/web_request_handler.h