Change endpoints lifecycle to use activate / start / stop / deactivate

Change servicemix-common in the following way:

• refactor the existing start / stop method into init / shutdown (along with the doStart and doStop methods)
• add new start / stop methods with empty implementation in the Endpoint class
• use these new init / start / stop / shutdown lifecycle in the service unit
• implement start / stop for the polling endpoints
• implement start / stop for the jms consumer endpoint (it should start / stop the listener container)
• implement start / stop for the http consumer (a stopped endpoint should return an 503 Service Unavailable http status code)