fix generated timestamp on retry

Author: luccailliau

lib/execution-options.js

@@ -498,7 +498,10 @@ class DefaultExecutionOptions extends ExecutionOptions {
       const generator = this._client.options.policies.timestampGeneration;
 
       if ( types.protocolVersion.supportsTimestamp(this._client.controlConnection.protocolVersion) && generator) {
-        result = generator.next(this._client);
+        if (this._generatedTimestamp === undefined) {
+          this._generatedTimestamp = generator.next(this._client);
+        }
+        result = this._generatedTimestamp;
       } else {
         result = null;
       }