Goal: Create the bare infrastructure for the classes responsible for monitoring a group of servers Round Trip Times. 

(1) Define the interface for the TopologyListener, a class that gets notified when a server's new RTT(Round Trip Time) is published.

(2) Introduce a ServerPingMonitor that manages which servers to monitor. Implement the functionality to remove and add servers to monitor. 

(3) Define the skeleton of the SingleServerPingMonitor class which is responsible for monitoring a single server and measuring its RTT values at uniform time intervals. 

The actual functionality for the SingleServerPingMonitor will be implemented and tested in SERVER-45051.