bug 1089375 - Switch to drf_cached_instances

drf_cached_reads moved to separate repo:

https://github.com/jwhitlock/drf-cached-instances
fix bug 1089375 - Refactor duplicated cache code

Move code for loading primary keys of related items into new function,
call from the loader and serializers.
Merge pull request #11 from jwhitlock/1089375_spin_out_drf_cached_reads

bug 1089375 - spin out drf_cached_reads