Updating config is not atomic

[rleungx]

Bug Report

pd/pkg/dashboard/adapter/manager.go

Lines 182 to 186 in a6e855e

	cfg := m.srv.GetPersistOptions().GetPDServerConfig().Clone()
	cfg.DashboardAddress = m.members[minMemberIdx].GetClientUrls()[0]
	if err := m.srv.SetPDServerConfig(*cfg); err != nil {
	log.Warn("failed to set persist options")
	}

Consider that we update the config using API at the same time, the timeline could be:

1. The dashboard first gets config and clone it
2. API finish updating config
3. The dashboard updates its config in the last

Step 2's updates could be overridden.