volume server adds ping function

weed/pb/volume_server.proto

@@ -107,6 +107,10 @@ service VolumeServer {
 
     rpc VolumeNeedleStatus (VolumeNeedleStatusRequest) returns (VolumeNeedleStatusResponse) {
     }
+
+    rpc Ping (PingRequest) returns (PingResponse) {
+    }
+
 }
 
 //////////////////////////////////////////////////
@@ -573,3 +577,10 @@ message VolumeNeedleStatusResponse {
     uint32 crc = 5;
     string ttl = 6;
 }
+
+message PingRequest {
+    string target = 1; // default to ping itself
+    string target_type = 2;
+}
+message PingResponse {
+}