Consider optimizing multi-key lookups

[maxim]

When wordmap is queried with multiple keys (e.g. wmap.query('a', 'b', 'c')) it currently bsearches each one independently, then optimizes the data reads based on the results. However, if instead of reading one key on every bsearch, we read 10kb worth of keys, that could take care of a bunch of other requested keys, and stop bsearching for others.