cubicweb #1625374 add alternative implementation of source.process_result with a generator [resolved]

for the portable database backup, we need to process all the rows of the different tables in the database, and having a generator for process_result will enable memory efficient processing of these rows.

done in3.12.4
load left0.000
closed by#c419c2d0d13e add a new method iter_process_result which does the same as proces_result but is a generator (closes #1625374)