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.
I'd expect an API which would let me create a PingManager, configure and only after that schedule the pingManager.
Currently it creates and schedule in one step - only after which I'm able to configure the PingManager to my needs.