Peer RPCs for bucket notifications (#2877)

* Implements a Peer RPC router that sends info to all Minio servers in the cluster.
* Bucket notifications are propagated to all nodes via this RPC router.
* Bucket listener configuration is persisted to separate object layer
  file (`listener.json`) and peer RPCs are used to communicate changes
  throughout the cluster.
* When events are generated, RPC calls to send them to other servers
  where bucket listeners may be connected is implemented.
* Some bucket notification tests are now disabled as they cannot work in
  the new design.
* Minor fix in `funcFromPC` to use `path.Join`

cmd/server-startup-msg.go

@@ -83,10 +83,10 @@ func printEventNotifiers() {
 		return
 	}
 	arnMsg := colorBlue("SQS ARNs: ")
-	if len(globalEventNotifier.queueTargets) == 0 {
+	if len(globalEventNotifier.external.targets) == 0 {
 		arnMsg += colorBold(fmt.Sprintf(getFormatStr(len("<none>"), 1), "<none>"))
 	}
-	for queueArn := range globalEventNotifier.queueTargets {
+	for queueArn := range globalEventNotifier.external.targets {
 		arnMsg += colorBold(fmt.Sprintf(getFormatStr(len(queueArn), 1), queueArn))
 	}
 	console.Println(arnMsg)