When the course category cache needs to be repopulated, two entries are added
to the cache for each category. In core this is currently done in a for-loop,
resulting in N set calls to the cache (where N is twice the number of
categories in Moodle).

This change switches that code to using a single set_many cache call instead,
which makes this an O(1) operation instead of O(N).

For a site with say, 1000 categories, this results in only 1 set call to the
cachestore instead of 2000.