Please provide a way for users to mmap databases with different mmap options or madvise settings

[rafl]

The mmapping done by this library is currently very opaque to the user. It's not easily possible to give the kernel any hints about memory management using the various mmap flags or madvise on the mmaped region.

Exposing options to do so might allow users to tune the libraries behaviour for their use case.

[oschwald]

We tried this but it ended up causing other issues.