FELIX-4252 Make Extender's ThreadPool size configurable

* Through "org.apache.felix.ipojo.extender.ThreadPoolSize" System property
* Through ConfigurationAdmin (PID: org.apache.felix.ipojo.extender.ExecutorQueueService)
* Added a ThreadGroup iPOJO Extender's threads

git-svn-id: https://svn.apache.org/repos/asf/felix/trunk@1527573 13f79535-47bb-0310-9956-ffa450edef68