middleware/proxy: Make Unhealthy a pointer (#615)

Pointer updates are atomic so drop the sync.RWMutex as it is not needed
anymore. This also fixes the race introduced with dfc71df (although I
believe this is the first time we properly tested that code path).

middleware/proxy/google.go

@@ -218,11 +218,11 @@ func newUpstream(hosts []string, old *staticUpstream) Upstream {
 			Conns:       0,
 			Fails:       0,
 			FailTimeout: upstream.FailTimeout,
-			Unhealthy:   false,
+			Unhealthy:   newBool(),
 
 			CheckDown: func(upstream *staticUpstream) UpstreamHostDownFunc {
 				return func(uh *UpstreamHost) bool {
-					if uh.Unhealthy {
+					if *uh.Unhealthy {
 						return true
 					}