Pointer Type for signal_handler

[nsoblath]

As @MCFlowMace mentioned in #91, we should probably not be passing std::shared_ptrs in add_cancelable() and remove_cancelable(). #91 (comment)

We should consider how signal_handler uses references to cancelables to decide what the right pointer type is for the function interface and for the storing. Let's use this thread to figure out what the right path is.

Considerations

For the most part, signal_handler should not own the cancelables, and should not control their lifetime or behavior with one exception: at some point determined by the user or client code, signal_handler will tell the cancelables it knows about and are valid to cancel their operations.

Not all cancelables are passed to signal_handler for cancelation responsibilities. Therefore cancelable itself cannot assume that it gets added to or removed from signal_handler at any given times (e.g. at construction and destruction, respectively).

Therefore signal_handler needs a way to store a non-owning reference to a subset of the cancelables that exist in an application at any given time. At some point signal_handler will need to use those references without dereferencing pointers to objects that have already been deleted.

Current reference storage

My solution to the storage question was to use weak_ptr. Actually, the reference are stored in a map< cancelable*, weak_ptr<cancelable> > since sets cannot be indexed by std::weak_ptrs. When signal_handler::cancel_all() is used, `weak_ptr::expired() is used to check whether the pointed-to object still exists before the pointer is dereferenced.

Current interface

Right now the add_cancelable() and remove_cancelable() functions take shared_ptr<cancelable> as their argument.

Suggestion

If the storage solution seems reasonable, then the main thing that probably needs changing is the interface arguments. They. shouldn't use shared_ptr because of the reasons @MCFlowMace mentioned. What if instead we pass weak_ptr<cancelable>? This would not imply ownership, and it would still pass the needed information.