types/persist: split controlclient.Persist into a small leaf package

This one alone doesn't modify the global dependency map much
(depaware.txt if anything looks slightly worse), but it leave
controlclient as only containing NetworkMap:

bradfitz@tsdev:~/src/tailscale.com/ipn$ grep -F "controlclient." *.go
backend.go:     NetMap        *controlclient.NetworkMap // new netmap received
fake_test.go:   b.notify(Notify{NetMap: &controlclient.NetworkMap{}})
fake_test.go:   b.notify(Notify{NetMap: &controlclient.NetworkMap{}})
handle.go:      netmapCache       *controlclient.NetworkMap
handle.go:func (h *Handle) NetMap() *controlclient.NetworkMap {

Once that goes into a leaf package, then ipn doesn't depend on
controlclient at all, and then the client gets smaller.

Updates #1278

ipn/ipnlocal/loglines_test.go

@@ -9,13 +9,13 @@ import (
 	"testing"
 	"time"
 
-	"tailscale.com/control/controlclient"
 	"tailscale.com/ipn"
 	"tailscale.com/ipn/ipnstate"
 	"tailscale.com/logtail"
 	"tailscale.com/tailcfg"
 	"tailscale.com/tstest"
 	"tailscale.com/types/key"
+	"tailscale.com/types/persist"
 	"tailscale.com/wgengine"
 )
 
@@ -67,7 +67,7 @@ func TestLocalLogLines(t *testing.T) {
 	}
 
 	// log prefs line
-	persist := &controlclient.Persist{}
+	persist := &persist.Persist{}
 	prefs := ipn.NewPrefs()
 	prefs.Persist = persist
 	lb.SetPrefs(prefs)