Port a bug fix from MySQL 5.7, so that all undo log pages will be freed
during a slow shutdown. We cannot scrub pages that are left allocated.

commit 173e171c6fb55f064eea278c76fbb28e2b1c757b
Author: Thirunarayanan Balathandayuthapani <thirunarayanan.balathandayuth@oracle.com>
Date:   Fri Sep 9 18:01:27 2016 +0530

    Bug #24450908   UNDO LOG EXISTS AFTER SLOW SHUTDOWN

    Problem:
    ========

    1) cached undo segment is not removed from rollback segment history
    (RSEG_HISTORY) during slow shutdown. In other words, If the segment is
    not completely free, we are failing to remove an entry from the history
    list. While starting the server, we traverse all rollback segment slots
    history list and make it as list of undo logs to be purged in purge
    queue.
    In that case, purge queue will never be empty after slow shutdown.

    2) Freeing of undo log segment is linked with removing undo log header
    from history.

    Fix:
    ====
    1) Have separate logic of removing the undo log header from
    history list from rollback segment slots and remove it from
    rollback segment history even though it is not completely free.
Reviewed-by: Debarun Banerjee <debarun.banerjee@oracle.com>
Reviewed-by: Marko Mäkelä <marko.makela@oracle.com>
    RB:13672