JENKINS-24272 Add onReconnect() callback to EngineListener

Add onReconnect() callback to EngineListener to allow jenkins to hook into a slave reconnect attempt.
Forms the basis of a fix for JENKINS-24272
"jnlp slaves fail to reconnect when master is restarted"

Once this is released corresponding changes to jenkins core are required to hook up onReconnect() instead of onDisconnect(). See separate jenkinsci pull request for details.
Good point. I added that.