client: fix races in use of goroutine group

The group utility struct does not support asynchronously launched
goroutines (goroutines-inside-of-goroutines), so switch those uses to a
normal go call.

This means watchNodeUpdates and watchNodeEvents may not be shutdown when
Shutdown() exits. During nomad agent shutdown this does not matter.

During tests this means a test may leak those goroutines or be unable to
know when those goroutines have exited.

Since there's no runtime impact and these goroutines do not affect alloc
state syncing it seems ok to risk leaking them.

client/client.go

@@ -1215,10 +1215,10 @@ func (c *Client) registerAndHeartbeat() {
 	c.retryRegisterNode()
 
 	// Start watching changes for node changes
-	c.shutdownGroup.Go(c.watchNodeUpdates)
+	go c.watchNodeUpdates()
 
 	// Start watching for emitting node events
-	c.shutdownGroup.Go(c.watchNodeEvents)
+	go c.watchNodeEvents()
 
 	// Setup the heartbeat timer, for the initial registration
 	// we want to do this quickly. We want to do it extra quickly
@@ -2600,6 +2600,7 @@ type group struct {
 	wg sync.WaitGroup
 }
 
+// Go starts f in a goroutine and must be called before Wait.
 func (g *group) Go(f func()) {
 	g.wg.Add(1)
 	go func() {
@@ -2608,6 +2609,8 @@ func (g *group) Go(f func()) {
 	}()
 }
 
+// Wait for all goroutines to exit. Must be called after all calls to Go
+// complete.
 func (g *group) Wait() {
 	g.wg.Wait()
 }