Previously the SIGNAL_COMPLETE would be sent before the result had
actually been placed in the result store. The docs indicate that the
signal is emitted when a task completes successfully, and completion
should imply that the result is ready.

Fixes #449

Refs #449

Previously, when using worker processes, a worker that was busy
executing a task would not terminate immediately even when SIGTERM was
used. This meant that, in-effect, all shutdowns had to be graceful. This
issue is now resolved. When SIGTERM is sent, any workers executing a
task will be stopped (as originally intended). Graceful shutdown
continues to function as it always has.

Fixes #452

This makes it a lot easier/obvious to subclass the desired
implementation and extend just what you want.