[Logging] Symlink to latest logs

[ShreyasKallingal]

Resolves #3617

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Any manual or new tests for this PR (please specify below)
• Relevant individual smoke tests: pytest tests/test_smoke.py::test_fill_in_the_name

New smoke test

pytest tests/test_smoke.py::test_symlink_latest_logs

Manual steps:

1. Run sky local up or some sky launch/exec command (doesn't matter if it succeeds or not -- just need to check log output)
2. Observe symlink "sky_latest" via ls -l ~/sky_logs
3. Repeat and confirm that the symlink points to the latest logs each time

[romilbhardwaj]

Thanks @ShreyasKallingal. Took a quick look and tested it a bit. Ran into an issue with sky launch on my mac, see comment for logs.

[romilbhardwaj]

Thanks @ShreyasKallingal! Tried it and works nicely. Left some comments on the code.

[romilbhardwaj]

Thanks @ShreyasKallingal! some minor comments, otherwise LGTM!

[Michaelvll]

Thanks for adding this @ShreyasKallingal! We want to take care of the concurrency issue below. :)

[ShreyasKallingal]

I grouped the symlink removal with creation in a critical section. And refactored to add a fast path for logging to /dev/null. Thanks for your review!