Introduce Query.Options object and add explain option (#36)

## What is the goal of this PR?
We introduce the Query `Options` message, which now contains `explain` flag on a query to ensure that the server knows when to cache explanations only when the user requests it.  Using `oneof` as the optional syntax is adopted instead of using the NULL message, which will be removed elsewhere as well in the future.

## What are the changes implemented in this PR?
* Add `explain` boolean option to `new Transaction.Query.Options`, that defaults to False
* Add `Transaction.Query.Options` that now wraps the `infer` and `explain` boolean flags

Author: flyingsilverfin

session/Session.proto

@@ -143,18 +143,23 @@ message Transaction {
         message Iter {
             message Req {
                 string query = 1;
-                INFER infer = 2;
-                // We cannot use bool for `infer` because GRPC's default value for bool is FALSE
-                // We use enum INFER instead, because the default value is index 0 (TRUE)
+                Transaction.Query.Options options = 2;
             }
             message Res {
                 Answer answer = 1;
             }
         }
-        enum INFER {
-            TRUE = 0;
-            FALSE = 1;
-            // The default value of this enum is 0 (TRUE)
+
+        message Options {
+            // optional infer flag
+            oneof infer {
+                bool inferFlag = 1;
+            }
+
+            // optional explain flag
+            oneof explain {
+                bool explainFlag = 2;
+            }
         }
     }