This commit modifies the signature of the `Progress` callback
so that its return value will indicate whether the request shall
continue to be processed by returning `true`, or if it shall
be aborted by returning `false`. Such modification will allow
one to cancel an ongoing request before it has completed.
When migrating, developers should modify there `Progress`
callbacks to always return `true` by default in case there
do not want to benefit from the cancelation feature.
A few unit tests use cases were provided, but anyone should feel
free to provide additional uses cases that they find relevant.
82fc7d5591
