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/routers.go

@@ -80,6 +80,9 @@ func configureServerHandler(srvCmdConfig serverCmdConfig) http.Handler {
 		registerDistNSLockRouter(mux, srvCmdConfig)
 	}
 
+	// Register S3 peer communication router.
+	registerS3PeerRPCRouter(mux)
+
 	// Register controller rpc router.
 	registerControlRPCRouter(mux, srvCmdConfig)