syncs: add Map.WithLock to allow mutations to the underlying map (#8101)

Some operations cannot be implemented with the prior API:
* Iterating over the map and deleting keys
* Iterating over the map and replacing items
* Calling APIs that expect a native Go map

Add a Map.WithLock method that acquires a write-lock on the map
and then calls a user-provided closure with the underlying Go map.
This allows users to interact with the Map as a regular Go map,
but with the gaurantees that it is concurrent safe.

Updates tailscale/corp#9115

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

syncs/syncs_test.go

@@ -7,7 +7,6 @@ import (
 	"context"
 	"io"
 	"os"
-	"sync"
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
@@ -189,19 +188,11 @@ func TestMap(t *testing.T) {
 
 	t.Run("LoadOrStore", func(t *testing.T) {
 		var m Map[string, string]
-		var wg sync.WaitGroup
-		wg.Add(2)
+		var wg WaitGroup
 		var ok1, ok2 bool
-		go func() {
-			defer wg.Done()
-			_, ok1 = m.LoadOrStore("", "")
-		}()
-		go func() {
-			defer wg.Done()
-			_, ok2 = m.LoadOrStore("", "")
-		}()
+		wg.Go(func() { _, ok1 = m.LoadOrStore("", "") })
+		wg.Go(func() { _, ok2 = m.LoadOrStore("", "") })
 		wg.Wait()
-
 		if ok1 == ok2 {
 			t.Errorf("exactly one LoadOrStore should load")
 		}