Add support for service start cancellation. This comes in the following forms:

• For synchronous services, the executing thread would be interrupted
• For services which implement a new CancellableService subinterface, the (single) cancel() method of that interface would be invoked (possibly only in async mode)
• An isCancelled() method would be added to StartContext which returns a boolean flag

Services which were successfully cancelled would call a cancelled() method which would put the service into FAILED state, possibly with a special subtype of StartException e.g. StartCancelledException which need not be logged.

In addition we should provide a simple facility to automatically track asynchronous task threads to allow them to be automatically interrupted on task cancel.