Add parallel option to Options (#125)

## What is the goal of this PR?
To allow clients to disable parallel execution, we add the `parallel_opt` to `Options.

## What are the changes implemented in this PR?
* add `parallel_opt` to `Options.

Author: flyingsilverfin

protobuf/options.proto

@@ -35,19 +35,22 @@ message Options {
     oneof explain_opt {
         bool explain = 3;
     }
+    oneof parallel_opt {
+      bool parallel = 4;
+    }
     oneof batch_size_opt {
-        int32 batch_size = 4;
+        int32 batch_size = 5;
     }
     oneof prefetch_opt {
-        bool prefetch = 5;
+        bool prefetch = 6;
     }
     oneof session_idle_timeout_opt {
-        int32 session_idle_timeout_millis = 6;
+        int32 session_idle_timeout_millis = 7;
     }
     oneof schema_lock_acquire_timeout_opt {
-        int32 schema_lock_acquire_timeout_millis = 7;
+        int32 schema_lock_acquire_timeout_millis = 8;
     }
     oneof read_any_replica_opt {
-        bool read_any_replica = 8;
+        bool read_any_replica = 9;
     }
 }