Move the provisionStart to the placement for better organization

[kerthcet]

What would you like to be cleaned:

placement and provisionStart are highly coupled today, if we start the provisioning, we have the start time, then we'll have the placement.

trackedPod {
	provisionStart time.Time

	// placement is what this pod was provisioned against. Two readers: the poll loop files
	// the ready duration under the same dimensions as the provision counters, and DeletePod
	// takes the region the instance is reachable in. Set only where provisionStart is armed.
	placement
}

So maybe a better way is to merge them together and make it a pointer. Storing them in the cache could lead to data loss like Pod restarted, but it's still fine since we can get them from the nodeClaim anyway.

Why is this needed:

[kerthcet]

/help