Commit dc9c5554 moved the final phase of
the redo log copying to the background thread. This would sometimes cause
too little redo log to be copied at the end of the backup. We would only
guarantee copying up to the latest redo log checkpoint. This would produce
a consistent backup, but it could refer to a too old point of time.

xtrabackup_copy_log(), xtrabackup_copy_logfile(): Add the parameter 'last'.

xtrabackup_backup_low(): Copy any remaining part of the log after the
backup threads have terminated.