adding basic grpc to volume server

weed/pb/volume_server.proto

@@ -0,0 +1,30 @@
+syntax = "proto3";
+
+package volume_server_pb;
+
+//////////////////////////////////////////////////
+
+service VolumeServer {
+    //Experts only: takes multiple fid parameters. This function does not propagate deletes to replicas.
+    rpc BatchDelete (BatchDeleteRequest) returns (BatchDeleteResponse) {
+    }
+}
+
+//////////////////////////////////////////////////
+
+message BatchDeleteRequest {
+    repeated string file_ids = 1;
+}
+
+message BatchDeleteResponse {
+    repeated DeleteResult results = 1;
+}
+message DeleteResult {
+    string file_id = 1;
+    int32 status = 2;
+    string error = 3;
+    uint32 size = 4;
+}
+
+message Empty {
+}