master supports grpc ping

weed/pb/master.proto

@@ -35,7 +35,8 @@ service Seaweed {
   }
   rpc ReleaseAdminToken (ReleaseAdminTokenRequest) returns (ReleaseAdminTokenResponse) {
   }
-
+  rpc Ping (PingRequest) returns (PingResponse) {
+  }
 }
 
 //////////////////////////////////////////////////
@@ -328,3 +329,10 @@ message ReleaseAdminTokenRequest {
 }
 message ReleaseAdminTokenResponse {
 }
+
+message PingRequest {
+  string target = 1; // default to ping itself
+  string target_type = 2;
+}
+message PingResponse {
+}