SE-30: MultiThreadedHttpConnectionManager.shutdownAll not getting call on http endpoint dispose causes ClassLoader Leak

_ Invoking MultiThreadedHttpConnectionManager.shutdownAll on connector dispose ONLY when running in embedded mode
_ There is no automated test for this: needs to be tested in standalone (where there is only one cleaner thread created per mule instance) and embedded (where there is a cleaner thread per app)
(cherry-picked from 1412d70)