Adding support for "mtime" using extended file metadata

[xelra]

rclone has solved the mtime problem by storing them as metada:

The modified time is stored as metadata on the object as X-Bz-Info-src_last_modified_millis as milliseconds since 1970-01-01 in the Backblaze standard. Other tools should be able to use this as a modified time.

If b2_fuse were to do exactly the same, it wouldn't just make the mount a lot better, but also be compatible with rclone.

Thank you very much for considering.

[falense]

This is definitely something B2Fuse should do, unfortunately I do not have the spare time to add this feature at the moment.

Best Sondre