Suro ping

This patch implements a simple ping class that periodically checks if a single suro-server is up or down.

We noticed that if there is only one suro-server and it goes down, suro-client's static load balancer marks that server as "down", and does not mark it as back "up" again if the server comes back online. This necessitates a restart of suro-client, and since we've embedded suro-client in a larger application, that is something we want to avoid.

Currently, suro-server uses a DummyPing class by default, which does nothing by design. There is ping functionality built into the load balancer already, so all this SuroPing class does is take advantage of that functionality in cases where the usual round-robining wouldn't apply.