[STORE] Make `file_switch` directory default using `mmapfs` / `niofs`

`mmapfs` is really good for random access but can have sideeffects if
memory maps are large depending on the operating system etc. A hybrid
solution where only selected files are actually memory mapped but others
mostly consumed sequentially brings the best of both worlds and
minimizes the memory map impact.
This commit mmaps only the `dvd` and `tim` file for fast random access
on docvalues and term dictionaries.
This should still be `mmapfs` (in both places) since that's what the name of the setting is, otherwise someone could set it to `mmap` and it wouldn't take effect.