tpc_begin(): Clear self._tbuf at the same time we clear self._seriald

so we're consistent with the cleanup that tpc_abort() does.

src/ZEO/ClientStorage.py

@@ -737,6 +737,7 @@ class ClientStorage:
             raise
 
         self._serial = id
+        self._tbuf.clear()
         self._seriald.clear()
         del self._serials[:]
         self._tbuf.clear()