Implement Fetch protocol using Document return type (#210)

## Release notes: usage and product changes

We implement Fetch queries, which return concept Document streams.

Author: flyingsilverfin

proto/answer.proto

@@ -33,41 +33,15 @@ message RowEntry {
     }
 }
 
-//
-//message Explainables {
-//    map<string, Explainable> relations = 1;
-//    map<string, Explainable> attributes = 2;
-//    map<string, Owned> ownerships = 3;
-//
-//    message Owned {
-//        map<string, Explainable> owned = 1;
-//    }
-//}
-//
-//message Explainable {
-//    string conjunction = 1;
-//    int64 id = 2;
-//}
-//
-//message ConceptMapGroup {
-//    Concept owner = 1;
-//    repeated ConceptMap concept_maps = 2;
-//}
-//
-//message ValueGroup {
-//    Concept owner = 1;
-//    optional Value value = 2;
-//}
-
-message ReadableConceptTree {
-
-    Node.Map root = 1;
+message ConceptDocument {
+
+    Node root = 1;
 
     message Node {
         oneof node {
             Map map = 1;
             List list = 2;
-            ReadableConcept readable_concept = 3;
+            Leaf leaf = 3;
         }
 
         message Map {
@@ -78,17 +52,29 @@ message ReadableConceptTree {
             repeated Node list = 1;
         }
 
-        message ReadableConcept {
-            oneof readable_concept {
-                EntityType entity_type = 1;
-                RelationType relation_type = 2;
-                AttributeType attribute_type = 3;
+        message Leaf {
+            oneof leaf {
+                Empty empty = 1;
+
+                EntityType entity_type = 10;
+                RelationType relation_type = 11;
+                AttributeType attribute_type = 12;
+                RoleType role_type = 13;
+                ValueType value_type = 15;
 
-                RoleType role_type = 4;
+                Attribute attribute = 20;
+                Value value = 21;
+
+                Kind kind = 30;
+            }
 
-                Attribute attribute = 5;
+            message Empty {}
 
-                Value value = 6;
+            enum Kind {
+                Entity = 0;
+                Relation = 1;
+                Attribute = 3;
+                Role = 4;
             }
         }
     }

proto/concept.proto

@@ -84,6 +84,7 @@ message Value {
         Datetime datetime = 1;
         oneof timezone {
             string named = 2;
+            // offset in seconds
             sint32 offset = 3;
         }
     }

proto/query.proto

@@ -32,20 +32,21 @@ message Query {
         message Ok {
             oneof ok {
                 Empty empty = 1;
-                ReadableConceptTreeStream readable_concept_tree_stream = 3;
+                ConceptDocumentStream concept_document_stream = 3;
                 ConceptRowStream concept_row_stream = 4;
             }
 
-            message Empty {
+            message Empty {}
 
-            }
-
-            message ReadableConceptTreeStream {
+            message ConceptDocumentStream {
                 // note: we could use this first response to record debug info, type annotations, warnings, etc
-                // TODO: Add Type query_type
+                // TODO: network optimisation: replace keys with IDs, sending keys in the header to rebuild the document on the client side
+                // TODO: network optimisation: replace types (== mostly constant strings) with a IDs, sending types in the header to rebuild on the client side
+                Type query_type = 2;
             }
 
             message ConceptRowStream {
+                // TODO: network optimisation: replace types (== mostly constant strings) with a IDs, sending types in the header to rebuild on the client side
                 repeated string column_variable_names = 1;
                 Type query_type = 2;
             }
@@ -54,12 +55,12 @@ message Query {
 
     message ResPart {
         oneof res {
-            ReadableConceptTreesRes trees_res = 1;
+            ConceptDocumentsRes documents_res = 1;
             ConceptRowsRes rows_res = 2;
         }
 
-        message ReadableConceptTreesRes {
-            repeated ReadableConceptTree trees = 1;
+        message ConceptDocumentsRes {
+            repeated ConceptDocument documents = 1;
         }
 
         message ConceptRowsRes {