[IMPORTANT] Fix potential race condition and improve code

[Tibimac]

Sometimes, when several requests are made simultaneously, a race condition could cause a crash.
This PR fix this problem and also improve the code by :

• using the property taskIdentifier to uniquely identify the URLSessionDataTask created
• removing both the data buffer and the handler from the dictionaries where they're stored when the request complete

[rinov]

Thanks for the fix! Cleaning up the buffers/handlers on completion makes the flow safer and more robust.