Ensure FileLock doesn't outlive the locked file

[bjorn3]

This could be done by either adding a lifetime parameter and storing BorrowedFd or by storing an owned File/OwnedFd and adding a Deref/DerefMut impl to FileLock.

cc #1651 (comment)

[squell]

Note: this requires a mild refactor in the timestamp code (since it is split over several methods that write via the File), so I'm hesitant to slap good first issue on it. (But it does seem like a relatively straightforward even though a bit tedious task)