Don't keep Extension instances

All instance of Extension extended class must be fully managed by
Jenkins instance. So those must not be reused by other instances.

This patch fixes such issue. Targets are:

* ToGerritRunListener
* DependencyQueueTaskDispatcher

Fix for JENKINS-24575

Task-Url: https://issues.jenkins-ci.org/browse/JENKINS-24575
Fix checkstyle error

This patch increase the threshold of file length.
Permanent fix is to be refactored, but it is out of scope for this
change.
Add spec test
Checks list is empty or not
Merge pull request #175 from rinrinne/extension-instances

Don't keep Extension instances