Fix JENKINS-25676 (and also JENKINS-25222)

Instead of going with an server specific solution, re-use Smack's PingManager for all connections.

Right now it's not possible to change the server ping interval. This feature can be added in a later version. Right now the change should address JENKINS-25676 and JENKINS-25222, but would be great if the affected users could confirm that.
Oh, yeah. I was confusing setPingInterval with setDefaultPingInterval.
Nevertheless, I find it confusing that a setter method has side effects (beyond changing the property value of course).
Using e.g. the builder pattern would make the API IMHO more readable and not more complex.