COM_CHANGE_USER may leave stale IDs to be checked

If a COM_CHANGE_USER is executed, the history of executed commands is cleared. If a backend connection was waiting for the command to be completed to check whether the result it got was correct, it was possible that the list of IDs to check was not pruned correctly. This resulted in a potential for very gradual growth in the history ID check list.

In practice this does not seem to be a problem unless a very large batch of session commands is executed after which a COM_CHANGE_user is done.