3.0 Add query type to all Query responses (#211)

## Release notes: usage and product changes
We add a `query_type` field to all the `Query.InitialRes.Ok` to support
retrieval of this information for any `QueryAnswer` on the client side.
Additionally, the `Ok.Empty` message was renamed to `Ok.Done`.

Author: farost

proto/query.proto

@@ -31,12 +31,14 @@ message Query {
 
         message Ok {
             oneof ok {
-                Empty empty = 1;
+                Done done = 1;
                 ConceptDocumentStream concept_document_stream = 3;
                 ConceptRowStream concept_row_stream = 4;
             }
 
-            message Empty {}
+            message Done {
+                Type query_type = 1;
+            }
 
             message ConceptDocumentStream {
                 // note: we could use this first response to record debug info, type annotations, warnings, etc