- Small adjustments for more optimial behavior
  - Revive IO burst at checkpoint around max_modified_age_sync
  - added bool option innodb_flush_sync = [ON|OFF] to choose the behavior.
    ON is the default to keep previous behavior for upgraded users

    static MYSQL_SYSVAR_BOOL(flush_sync, srv_flush_sync,
    PLUGIN_VAR_NOCMDARG,
    "Allow IO bursts at the checkpoints ignoring io_capacity setting.",
    NULL, NULL, TRUE);

  Reviewed by: Sunny Bains (sunny.bains@oracle.com)
  RB: 7849
  Created by: Yasufumi