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.
> 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?

No, setPingInterval takes effect immediately.

> I'd expect an API which would let me create a PingManager, configure it and only after that schedule it.
> Currently it creates and schedule in one step - only after which I'm able to configure the PingManager to my needs.

Since it takes effect immediately, this there is no need to do so and it would result in a more complex API as more methods would be needed (`configureServerPing`, `startServerPing`).