Enforce 12 hour maxJobDuration

[brendanmcfarland]

Opening a pr to close the loop on timeout handling.

• we can't extend message visibility past 12 hours
• we don't have a single source of truth to avoid duplicate work

As it stands right now watchbot will duplicate work if the message takes longer than 12 hours to process and no timeout is set. Quick solution is to acknowledge the current 12 hour limitation by enforcing maxJobDuration on workers.

I've chosen 43020 as the max (12 hours minus three minutes) to avoid edge cases that could occur if the heartbeat tries to extend the visibility timeout 3 minutes and the total would be over 43200 seconds. Also attempted some docs to explain this limitation.

cc: @jakepruitt @rclark

closes #203