message: Clear cached history object in commit_and_continue_as_read
  Otherwise we can continue to use the single write history
  object although we are in a read transaction.
diff: @@ -2038,6 +2038,7 @@ DB::version_type Transaction::commit_and_continue_as_read()
  // Remap file if it has grown, and update refs in underlying node structure
  remap_and_update_refs(m_read_lock.m_top_ref, m_read_lock.m_file_size, false); // Throws
  + m_history = nullptr;
  set_transact_stage(DB::transact_Reading);
  return version;
  