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/globals.go

@@ -52,8 +52,13 @@ var (
 	globalMaxCacheSize = uint64(maxCacheSize)
 	// Cache expiry.
 	globalCacheExpiry = objcache.DefaultExpiry
+	// Minio local server address (in `host:port` format)
+	globalMinioAddr = ""
 	// Minio default port, can be changed through command line.
 	globalMinioPort = 9000
+	// Peer communication struct
+	globalS3Peers = s3Peers{}
+
 	// Add new variable global values here.
 )