make Throttle adaptive to rate not rate + 1mbit

Setting the Throttle to rate +1 mbit has some bad potential.

Assume we have a file with a long initial credits and long ending, many slow sceenes with less bitrate. And one long lasting high bitrate scene. If the scenes bitrate is higher than the rate + 1 mbit / cache size, the file needs to buffer again.
By setting it to rate * 2, we will most likely not get any buffering.

Why do we set a Throttle anyway? To keep others amused and don't take the whole bandwidth?

I often have problems with buffering over WLAN, but this PR helps in my case.