Add per-topic message rate limiting mechanism.

	modified:   src/bruce/anomaly_tracker.cc
	modified:   src/bruce/anomaly_tracker.h
	modified:   src/bruce/conf/conf.cc
	modified:   src/bruce/conf/conf.h
	modified:   src/bruce/conf/conf.test.cc
	new file:   src/bruce/conf/topic_rate_conf.cc
	new file:   src/bruce/conf/topic_rate_conf.h
	modified:   src/bruce/discard_file_logger.cc
	modified:   src/bruce/discard_file_logger.h
	modified:   src/bruce/msg.cc
	modified:   src/bruce/msg.h
	new file:   src/bruce/msg_rate_limiter.cc
	new file:   src/bruce/msg_rate_limiter.h
	new file:   src/bruce/msg_rate_limiter.test.cc
	modified:   src/bruce/router_thread.cc
	modified:   src/bruce/router_thread.h
	modified:   src/bruce/scripts/check_bruce_discards.py
	modified:   src/bruce/util/time_util.cc
	modified:   src/bruce/util/time_util.h
	modified:   src/bruce/web_request_handler.cc