Summary:
Some workloads (like my current benchmarking) may want partitioned indexes without partitioned filters. Particularly, when `-optimize_filters_for_hits=true`, the total index size may be larger than the total filter size, so it can make sense to hold all filters in-memory but not all indexes.
Closes https://github.com/facebook/rocksdb/pull/3492
Differential Revision: D6970092
Pulled By: ajkr
fbshipit-source-id: b7fa1828e1d13829339aefb90fd56eb7c5337f61
0a0fad447b