Add mtime column toggle

[alexdotc]

Notes / possible changes

1. Clippy is mad about a 9 argument function
2. It's possible there's a much slicker way to handle mtime parsing that involves less type conversions
3. Maybe we can/should just calculate the current time once at startup for the format string comparison?

[alexdotc]

@lgarrison any criticism welcome - this adds a togglable/sortable column in the format style of ls -l

Separately, ceph also has an rctime xattr that might be useful

[lgarrison]

Looks great! Have you explored the rctime xattr at all? I assume it's like a recursive mtime, which could potentially be more intuitive, since all the other attributes we display are recursive. On the other hand, some old ceph bug reports seem to suggest it's not reliable or at least not updated promptly.

[alexdotc]

I went ahead and swapped mtime out for ctime. We could provide both, but it seems slightly overloaded for now; we can always add it back in. I added a note to --help but I'm not sure if that's the best way to expose things. The note will be easier to split up and this might make a bit more sense with flags to control default sort/toggle on startup (sort of mentioned in #11)

It's a bit wider than the scope of this branch, but I also swapped out stat size for xattr size and did a bit of refactoring on the xattr calls. We now do 6 getxattr() calls per directory inode, but at least this should take advantage of MDS caching, which is probably the bigger consideration. And hardcoding xattr sizes per #7 will cut the number of calls in half.

[alexdotc]

@lgarrison If we merge this, I'll try and do this before we cut a release:

And hardcoding xattr sizes per #7 will cut the number of calls in half.

[lgarrison]

Looks good! I tested it locally and it works great.