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.
Yes there is, not all Smack instances run on systems with a wired connection. For example, some run on Android, which knows different types of connectivity requiring different ping intervals. Destroying the old one and creating a new one would mean additional LOCs and would make the API harder to use IMHO.