Repeated file access via share-token race

Race condition in ShareHandler in Patrick Hener goshs before 2.1.0 allows remote attackers to read shared files repeatedly via parallel token requests. ShareHandler checks DownloadLimit and Downloaded under a read lock, releases that lock while serving the file, and increments Downloaded only after the transfer, so concurrent goroutines can all observe the same pre-increment state and each pass the limit check. Exploitation requires possession of a valid share token, but no account or further user interaction; a nominally single-use or limited-use link can therefore be redeemed more times than the operator intended.