Provide access to the shutdown task Future in order to have more control over it

I'm submitting this patch in order to have a greater control over the shutdown task.
It's very useful when integrating Camel in another product. Therefore you can have a user interface for controlling the life-cycle of a context. The user can start or stop a context and if it takes too much time to stop he can force it by calling getCurrentShutdownFuture().cancel(true)