camel-catalog-maven - Add stop method to cleanup connections

Make it possible to cleanup the http connections that MultiThreadedHttpConnectionManager may leak.

For example from testing camel-idea-plugin

Failed tests:
CamelProjectComponentTestIT>PlatformTestCase.v:640->ModuleTestCase.tearDown:79->IdeaTestCase.tearDown:49->PlatformTestCase.tearDown:517->PlatformTestCase.q:503 Thread leaked: Thread[MultiThreadedHttpConnectionManager cleanup,6,main]; WAITING (true)
— its stacktrace:
at java.lang.Object.wait(Native Method)
at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:143)
at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:164)
at org.apache.commons.httpclient.MultiThreadedHttpConnectionManager$ReferenceQueueThread.run(MultiThreadedHttpConnectionManager.java:1122)
—