LP #1582345: Handle MLOG_INDEX_LOAD during the backup

**Reported in Launchpad by Sergei Glushchenko last update 17-05-2016 07:09:57

MySQL 5.7 can sometimes skip redo logging when creating index. If such ALTER TABLE being issued during the backup, the backup will be inconsistent. xtrabackup has to detect such DDL and abort the backup. The only way to do so is to parse log file while copying and abort if MLOG_INDEX_LOAD has been found.