Make simulator work with large scale cluster

[rleungx]

Enhancement Task

Currently, the store heartbeat needs to wait for all regions to have been processed. So the store will change to disconnected from the PD perspective. See

pd/tools/pd-simulator/simulator/drive.go

Lines 136 to 147 in b3bd6da

	// Tick invokes nodes' Tick.
	func (d *Driver) Tick() {
	d.tickCount++
	d.raftEngine.stepRegions()
	d.eventRunner.Tick(d.tickCount)
	for _, n := range d.conn.Nodes {
	n.reportRegionChange()
	d.wg.Add(1)
	go n.Tick(&d.wg)
	}
	d.wg.Wait()
	}

We need to separate these two logic.