- torrent.go: Add nil cache check in expired() and protect against double close
- piece.go: Remove isClosed check in Release() to prevent library deadlock
- apihelper.go: Wait for torrent close via closedChan before deleting files
- cache.go: Fix deadlock in getRemPieces() (copy readers without locking)
- cache.go: Add TryLock() in cleanPieces() to prevent recursive calls