Parallelize instance startup on daemon start

[stgraber]

Is there an existing issue for this?

• There is no existing issue for this feature

What are you currently unable to do

When starting systems with thousands of VMs, the fact that instance startup on daemon start is completely sequential isn't exactly ideal.

It would make more sense to use the parallel logic found in bulk instance action (incus start --all) in this situation too.

The main complication is the support for the priority and delay boot options on the instances.

As an easy compromise, I think we should:

• Locate all projects where all instances within the project have neither boot.autostart.priority nor boot.autostart.delay configured.
• Start all of those using logic similar to that from the bulk instance start logic.
• Then process the rest with the current sequential logic.

That should give us a fast path covering most situations while keeping support for those config options.

What do you think would need to be added

No response