health: add Tracker type, in prep for removing global variables

This moves most of the health package global variables to a new
`health.Tracker` type.

But then rather than plumbing the Tracker in tsd.System everywhere,
this only goes halfway and makes one new global Tracker
(`health.Global`) that all the existing callers now use.

A future change will eliminate that global.

Updates #11874
Updates #4136

Change-Id: I6ee27e0b2e35f68cb38fecdb3b2dc4c3f2e09d68
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/magicsock/magicsock_test.go

@@ -3120,14 +3120,14 @@ func TestMaybeSetNearestDERP(t *testing.T) {
 
 			report := &netcheck.Report{PreferredDERP: tt.reportDERP}
 
-			oldConnected := health.GetInPollNetMap()
+			oldConnected := health.Global.GetInPollNetMap()
 			if tt.connectedToControl != oldConnected {
 				if tt.connectedToControl {
-					health.GotStreamedMapResponse()
-					t.Cleanup(health.SetOutOfPollNetMap)
+					health.Global.GotStreamedMapResponse()
+					t.Cleanup(health.Global.SetOutOfPollNetMap)
 				} else {
-					health.SetOutOfPollNetMap()
-					t.Cleanup(health.GotStreamedMapResponse)
+					health.Global.SetOutOfPollNetMap()
+					t.Cleanup(health.Global.GotStreamedMapResponse)
 				}
 			}