Supporting mod time

[slifty]

rclone, by default, uses size and mod time as ways of verifying that two files are the same (maybe checksum too but the mod time is the point of this issue).

Permanent doesn't have an explicit concept of "mod time" associated with a file.

This means that rclone's checks after file upload fail -- it "updates the mod time" (which does nothing) and then checks the mod time (which has not been updated / is simulated from some other file date), and so the modtime does not generally equal the expected value.

We have two options:

1. Do not support default rclone; require users to set the --size-only and --sftp-set-modtime=false flags in order to sync with permanent

2. Add support for file modtime to permanent's backend (and API and SDK). Modtime would become a nullable attribute of file. We would need an endpoint to allow setting of a modtime for a given file ID.

The second is the right solution; the first involves the least (i.e. no) development time.