update on the low-memory patch

[Kostas Chatzikokolakis]

Hello,

I've been using the latest version of the low-memory patch in all
production systems I manage as well as for my personal backup for more
than a month. So far it works perfectly, using very low memory and
finishing in all cases where I was previously getting out of memory
errors. Also the issues I was having with files being concurrently
updated are gone now. Restore has also been tested several times.

Since other people were also experiencing memory issues I think it's
worth the effort to consider the patch for merge. More testing by other
people is very welcome.

Cheers,
Kostas