Use a separate key for caching prefetched filter rels

Use a separate key for caching prefetched filter rels

This silences the extra deprecation warnings about prefetched filter
rels vs. get(_inflated)_columns. XXX: Determine if these warnings are
actually valid and missed before, or bogus and should be silenced.

The warnings were coming from:

t/inflate/hri_torture.t line 25
t/multi_create/cd_single.t line 12
t/multi_create/diamond.t line 25
t/multi_create/existing_in_chain.t line 70
t/multi_create/in_memory.t line 80
t/multi_create/in_memory.t line 96
t/multi_create/in_memory.t line 101
t/multi_create/standard.t line 80
t/multi_create/standard.t line 115
t/multi_create/standard.t line 182
t/multi_create/torture.t line 21
t/multi_create/standard.t line 456
t/prefetch/manual.t line 27
t/prefetch/multiple_hasmany_torture.t line 13
t/relationship/custom_opaque.t line 11
t/resultset/inflate_result_api.t line 24