[hmr] Make the change-detecting behavior consistent

[CNSeniorious000]

Currently, changes made during a hot-reload trigger an immediate reload afterward. However, changes during the first run won't trigger a reload upon completion, as the watchfiles watcher only starts after the first run to reduce startup overhead.

I've thought 2 ways to implement the desired consistent behavior:

1. Start watchfiles from the beginning from a daemon thread (this may cause segmentfault on Linux when shutdown, see hmr-daem/v0.4)
2. Run stat checks before starting watchfiles. Blocked by [hmr] Support more watcher backends #410

[linear]

PROM-62 [hmr] Make the change-detecting behavior consistent