Allow specifying separate path for xtrabackup_logfile

It would be great if we could specify a separate path (mountpoint) for xtrabackup_logfile, and when backup completes it would copy to --dest like it's done for streaming backups. The idea is to allow xb to better keep up with server writes and avoid the classic error:

xtrabackup: error: log block numbers mismatch:
xtrabackup: error: expected log block no. 548490976, but got no. 556682968 from the log file.
xtrabackup: error: it looks like InnoDB log has wrapped around before xtrabackup could process all records due to either log copying being too slow, or log files being too small.
xtrabackup: Error: xtrabackup_copy_logfile() failed.

Something like --xtrabackup-logfile-dest=/path/to/speedy/drive/