The `FileCache` now uses `std::sync::Condvar` to signal when files have
finished loading. This replaces the previous spin-wait mechanism,
significantly improving efficiency by allowing waiting threads to sleep
until explicitly woken up.
The `wait_for_load` method now blocks on the `Condvar`, and the
`insert_m1` and `insert_tick` methods call `notify_all` after updating
the cache and removing the loading guard. This ensures that all waiting
threads are woken up and can re-check the cache.
Additionally, `ensure_m1_loaded` and `ensure_tick_loaded` in
`DataServer` have been simplified to directly call `wait_for_load` when
another thread is already loading the file, removing the redundant
synchronous loading logic.
Refactor FileCache to use Condvar
