egg-task-cache: Fix cancellation when tasks are queued for a given key

The callers' GCancellables are monitored, and whenever one is cancelled
it is removed from the queue. A separate GCancellable is used with the
in-flight GTask that fetches the value. Fetching is aborted if, and
only if, all the queued tasks for a given key have been cancelled.

https://bugzilla.gnome.org/show_bug.cgi?id=779660