Speed up pd leader server ready

[bufferflies]

Enhancement Task

pd/server/cluster/cluster.go

Lines 387 to 394 in 45bc455

	if c.isAPIServiceMode {
	// bootstrap keyspace group manager after starting other parts successfully.
	// This order avoids a stuck goroutine in keyspaceGroupManager when it fails to create raftcluster.
	err = c.keyspaceGroupManager.Bootstrap(c.ctx)
	if err != nil {
	return err
	}
	}

Currently, we need to reload all keyspaces metadata before the PD server is ready to serve. If the keyspaces is too many, it will take too long