Too many peers may cause rule-checker to be stuck by fitRegion

[JmPotato]

Bug Report

#6662 also exposes another problem. When too many peers are replicated to the redundant TiKV stores, the following code of rule-checker will generate a huge number of loops, resulting in its almost stuck and unable to complete the patrol of the region for a long time.

pd/pkg/schedule/placement/fit.go

Lines 269 to 283 in ac31f87

	limit := uint(1<<len(candidates) - 1)
	binaryInt := uint(1<<count - 1)
	for ; binaryInt <= limit; binaryInt++ {
	// there should be exactly `count` number in current binary number `binaryInt`
	if bits.OnesCount(binaryInt) != count {
	continue
	}
	selected := pickPeersFromBinaryInt(candidates, binaryInt)
	better = w.compareBest(selected, index) || better
	// reset the selected items to false.
	unSelectPeers(selected)
	if w.exit {
	break
	}
	}

If a region owns 36 peers, the limit here will reach 68719476735. We may need to optimize this kind of case to make sure fitRegion could be finished in a reasonable time.

[nolouch]

it should be fixed by #6660, and I think this will be a performance improvement, so I remove the bug label.