Recovery (experimental)

* Recovery script
For now it only supports in group recovery (aka merge)
* C and C++ bits for managing iteration results.
For now we support trivial append, extract, qsort(3)-based sorting and diff'ing (right outer flavor).

PS. There are still a lot of XXX: comments that needs to be dealt with before production use.
PPS. Tests are still on the way so code is only "eye-tested"