On a large site, the task to transfer question categories to the new
mod_qbanks contexts could take a very long time, as of any transferred
categories contain questions using files, each file has to be updated
individually.
This change splits up the transfer into 2 stages. The first stage
(transfer_question_categories) updates the context IDs on each question
category and moves them to their new top category, then spawns an
instance of transfer_questions for each category that was moved.
transfer_questions then handles moving the files and tags for questions
in a single category at a time. This allows the transfer process to be
parallelised by using multi cron or ad-hoc task runners, so should speed
things up on larger sites.
Mark Johnson
60f58afc5f