Fixed memory leak in Promise#cancellableGet

[mpilquist]

The old implementation of cancellableGet had two flaws:

• each invocation of force would register a callback under an internal id, overwriting any previously registered callback for the same id
• cancel would simply remove the registration for the id

For example, cancel *> force would result in a state where a callback for the force was registered. force *> force *> force would result in a similar state, where the first two force actions would never be completed.

To fix, the internal state has been changed so that we track a list of callbacks associated with each id. Calling cancellableGet registers an empty list under that id. Each force adds a new callback to the list, but only if that id is still in the map. Calling cancel removes the id from the map.

[mpilquist]

Fixes #1026

[SystemFw]

Nice idea