[Yundi339]

Harden concurrent access to the HTTP FileCache without changing its public size configuration.

Changes:

• serialize cold loads and refreshes for the same path;
• build refreshed entries off-cache before publishing them;
• protect mutable cache entry state with an entry mutex;
• keep cache entries alive while HTTP response buffers are being sent;
• skip entries currently in use during expiration cleanup;
• add a concurrent same-path regression test.

This is intentionally split from the larger FileCache configurability work in PR #823.