add topic/connection statistics

This patch adds a module to roscpp/rospy that monitors some metrics
over every ROS connection (period, dropped messages, traffic, age)
and periodically sends out aggregated statistics data over a common
topic /statistics. It is designed to be lightweight so it can be used
in "production" environments.

Statistics monitoring is disabled by default. It has to be enabled
explicitly using "rosparam set enable_statistics true".