Seek issue，use too much time.

```
private boolean isUseCache(GetRequest request) throws ProxyCacheException {
long sourceLength = source.length();
boolean sourceLengthKnown = sourceLength > 0;
long cacheAvailable = cache.available();
// do not use cache for partial requests which too far from available cache. It seems user seek video.
// return !sourceLengthKnown || !request.partial || request.rangeOffset <= cacheAvailable + sourceLength * NO_CACHE_BARRIER;
return !sourceLengthKnown || !request.partial || request.rangeOffset <= cacheAvailable;// + sourceLength * NO_CACHE_BARRIER;
}
```

HTTP MP4 file, if the duration too long, "sourceLength * NO_CACHE_BARRIER" well be a **BIG number**, fetch cache from network will coast too much time, seek blocked.
Remove the added number, seek works normally.

Search the repository for isUseCache(GetRequest request) and inspect how rangeOffset, cache.available(), and source.length() affect partial requests. Reproduce or review the HTTP MP4 seek scenario, then verify that seeking a long video does not wait for an unnecessarily large cache range.

Scritto dal modello di indicizzazione a partire dal testo della issue.