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.
Here's a thing I don't understand about the PingManager API (and also the reason why I implemented it without PingManager originally): 
If I use it this way PingManager would initially schedule itself with it's internal default ping interval and only after it has run for the 1st time it would use the interval I give it via setPingInterval, wouldn't it?