feat(pause): pausing immidiately instead of polling

[emjburns]

Pushing pause messages instead of updating the context then pausing on next poll of tasks.
Cascading pause message from execution -> running stages -> running tasks, instead of previous organization (update execution -> next time a task is run & execution paused, pause task -> pause stage tasks belongs to).

@robfletcher PTAL

[robfletcher]

This all looks very good and soundly implemented.

My only concern is whether there's a possibility of a race condition between the PauseTaskHandler and activity going on in RunTaskHandler. Consider, if RunTaskHandler has started doing something and got past the check for PAUSED status but concurrently another Orca instance processes a PauseTask message, the PAUSED status may subsequently get overwritten when the RunTaskHandler completes and sends a CompleteTask message.

It would not be an issue if RunTaskHandler re-queued its message, only if it pushes a CompleteTask message.

I'm not sure what the best solution is, though.

[emjburns]

Closing because of the possibility of race conditions. Instead, opening #1676 to implement this desired behavior.