bugfix: poolWatcher.Unregister() data race

You need to protect the poolWatcher.unregistered variable to avoid
the data race. It does not look too critical because we don't
expect that performance of poolWatcher.Unregister() is matter
in cuncurrent calls case.

It could also lead to crashes.

Part of #218

Author: oleg-jukovec

connection_pool/watcher.go

@@ -33,7 +33,7 @@ func (c *watcherContainer) remove(watcher *poolWatcher) bool {
 	if watcher == c.head {
 		c.head = watcher.next
 		return true
-	} else {
+	} else if c.head != nil {
 		cur := c.head
 		for cur.next != nil {
 			if cur.next == watcher {
@@ -85,7 +85,11 @@ type poolWatcher struct {
 
 // Unregister unregisters the pool watcher.
 func (w *poolWatcher) Unregister() {
-	if !w.unregistered && w.container.remove(w) {
+	w.mutex.Lock()
+	unregistered := w.unregistered
+	w.mutex.Unlock()
+
+	if !unregistered && w.container.remove(w) {
 		w.mutex.Lock()
 		w.unregistered = true
 		for _, watcher := range w.watchers {