[RAPPS] speed up hash-checking by doing it simultaneously to download

Currently we download the whole file synchronously and then afterwards we do the hash-checking.

It would be faster (and we could prevent that 100% CPU-usage spike at the end) if we would read into a buffer, do the (partial) hash-checking on that and then write buffers data into file.

This way 100% download progress will be almost in time with 100% hash-checked.