client: avoid registering node twice right away

I noticed that `watchNodeUpdates()` almost immediately after
`registerAndHeartbeat()` calls `retryRegisterNode()`, well after 5
seconds.

This call is unnecessary and made debugging a bit harder.  So here, we
ensure that we only re-register node for new node events, not for
initial registration.

client/util.go

@@ -3,6 +3,7 @@ package client
 import (
 	"fmt"
 	"math/rand"
+	"time"
 
 	"github.com/hashicorp/nomad/nomad/structs"
 )
@@ -63,3 +64,13 @@ func shuffleStrings(list []string) {
 		list[i], list[j] = list[j], list[i]
 	}
 }
+
+// stoppedTimer returns a timer that's stopped and wouldn't fire until
+// it's reset
+func stoppedTimer() *time.Timer {
+	timer := time.NewTimer(0)
+	if !timer.Stop() {
+		<-timer.C
+	}
+	return timer
+}