Add `mmap` option instead of loading the index

[jermp]

We could add a mmap option instead of loading the index for querying. That is, the user can specify:

1. --load (default choice): this loads the entire index in memory, assuming there is enough RAM for it. This is the intended usage if there is enough RAM and the queries are many.
2. --mmap: this memory maps the index file on disk, so that queries will fetch disk pages automatically. This is the intended usage when option --load is not used, i.e., when there isn't enough RAM or queries are sporadic.