[Segment Cache] Add CacheStatus.Empty (#73667)
This is a small refactor to allow creating a cache empty entry without
also triggering a server request. Currently these are combined into the
same phase, because there's no case where one operation happens without
the other.
However, I need to implement additional prefetching strategies. For
example, sometimes a segment's data will already be available as part of
a different server response.
To support this, I've split the Pending CacheStatus into two separate
fields:
- Empty: The cache entry has no data, and there's no pending request to
fetch it.
- Pending: The cache entry has no data, and there _is_ a pending request
to fetch it.
This is a refactor only, so there should be no change to external
behavior.
