[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.
Should this still be `"fileswitch(...)"` since it has been renamed to `default`? How about DefaultFileSwitchDirectory and returning the string `"default(...)"`?