net/netmon: factor out debounce loop, simplify polling impl

This simplifies some netmon code in prep for other changes.

It breaks up Monitor.debounce into a helper method so locking is
easier to read and things unindent, and then it simplifies the polling
netmon implementation to remove the redundant stuff that the caller
(the Monitor.debounce loop) was already basically doing.

Updates #9040

Change-Id: Idcfb45201d00ae64017042a7bdee6ef86ad37a9f
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/netmon/polling.go

@@ -13,7 +13,6 @@ import (
 	"sync"
 	"time"
 
-	"tailscale.com/net/interfaces"
 	"tailscale.com/types/logger"
 )
 
@@ -68,22 +67,18 @@ func (pm *pollingMon) Receive() (message, error) {
 		// so this can go very slowly there, to save battery.
 		// https://github.com/tailscale/tailscale/issues/1427
 		d = 10 * time.Minute
-	}
-	if pm.isCloudRun() {
+	} else if pm.isCloudRun() {
 		// Cloud Run routes never change at runtime. the containers are killed within
 		// 15 minutes by default, set the interval long enough to be effectively infinite.
 		pm.logf("monitor polling: Cloud Run detected, reduce polling interval to 24h")
 		d = 24 * time.Hour
 	}
-	ticker := time.NewTicker(d)
-	defer ticker.Stop()
-	base := pm.m.InterfaceState()
+	timer := time.NewTimer(d)
+	defer timer.Stop()
 	for {
-		if cur, err := pm.m.interfaceStateUncached(); err == nil && !cur.EqualFiltered(base, interfaces.UseInterestingInterfaces, interfaces.UseInterestingIPs) {
-			return unspecifiedMessage{}, nil
-		}
 		select {
-		case <-ticker.C:
+		case <-timer.C:
+			return unspecifiedMessage{}, nil
 		case <-pm.stop:
 			return nil, errors.New("stopped")
 		}