Add mixable_bit_index_storage and use it from lsh and minhash recommender
Add mixable_inverted_index_storage and use it from inverted_index recommender
Add mixable_lsh_index_storage and use it from euclid_lsh recommender
Remove mixable models from recommender driver and use those in algorithms.

This change removes public APIs of recommender_base for accessing internal storage (or table).

Note: anomaly::lof_storage has not adapted to this change yet: I wrote workaround.
Note: recommenders still use diff of type std::string. We should fix it.