Improve mmap support

[hayley-leblanc]

SquirrelFS currently uses a rudimentary implementation of mmap that does the bare minimum to achieve functionality without any optimizations. Other file systems (NOVA, WineFS) have optimizations to ensure consistency of memory-mapped data, improve memory-mapped access performance, etc.; we could implement similar (or new) improvements to give SquirrelFS better memory-mapped access performance and improve support for developers trying to write crash-consistent applications with mmap'ed files.