ipn/ipnlocal: make active IPN sessions keyed by sessionID

We used a HandleSet before when we didn't have a unique handle. But a
sessionID is a unique handle, so use that instead. Then that replaces
the other map we had.

And now we'll have a way to look up an IPN session by sessionID for
later.

Updates tailscale/corp#17859

Change-Id: I5f647f367563ec8783c643e49f93817b341d9064
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/local_test.go

@@ -37,7 +37,6 @@ import (
 	"tailscale.com/util/dnsname"
 	"tailscale.com/util/mak"
 	"tailscale.com/util/must"
-	"tailscale.com/util/set"
 	"tailscale.com/util/syspolicy"
 	"tailscale.com/wgengine"
 	"tailscale.com/wgengine/filter"
@@ -765,9 +764,6 @@ var _ legacyBackend = (*LocalBackend)(nil)
 
 func TestWatchNotificationsCallbacks(t *testing.T) {
 	b := new(LocalBackend)
-	// activeWatchSessions is typically set in NewLocalBackend
-	// so WatchNotifications expects it to be non-empty.
-	b.activeWatchSessions = make(set.Set[string])
 	n := new(ipn.Notify)
 	b.WatchNotifications(context.Background(), 0, func() {
 		b.mu.Lock()