When shutting-down gimp-parallel, cancel and/or abort any ongoing
and queued async operations, instead of finishing them (async
operations that already started executing will be canceled, but
execution will be blocked until they're finished.)  This is
especially important since we're shutting down gimp-parallel before
the destruction of data factories.  This commit causes any ongoing
async operations of the factories to be canceled on shutdown,
rather than waiting for them to finish normally.