Implement run pipeline

[r4victor]

Part of #3551

This PR implements RunPipeline that supersedes process_run background task. No major behavioral changes. One minor change is that service scaling now doesn't take place if rolling deployment is in progress (both upscaling and downscaling). Previously, only downscaling was skipped. This is just to make service processing steps independent and avoid hard-to-reason-about interactions.

Tested:

• Rolling deployment
• Autoscaling
• Multi-node tasks partial failure
• Run retry
• Scheduled tasks