Double file cache fix. Fixes ticket #15500

Double caching for e.g. dvdplayer has been broken for a long time. The actual problem for double cache was that when the cache was full, we still kept reading from the source which get disposed due to seek-requests which in turn also caused seek backs in the stream (which are really slow on e.g. http). This revealed itself while playing mp4 files with high a/v offset but generally this should improve performance (especially on slow systems) for all cached files. Furthmore we would block in WaitForData() when seeking just "over" the cache while it was full.

Would be great if this could be included for Helix.
That doesn't work. Resetting it on a cache switch where the new cache is full while heavy seeking causes it to screw up the average calculation. If you don't reset it here, the spikes will be averaged out. You can only recalculate average properly when cache is empty (like on a full reset). Bottomline: since overall we're only interested in the average rate over a longer period of time, without caring about heavy seeks and/or cache swaps, this seems like to most reliable approach.