YouTube can now request custom cache/prefetch parameters and disconnect-at-highwater

through the use of pseudo http headers specified in the setDataSource call.

x-cache-config: -1/-1/0
x-disconnect-at-highwatermark: 1

turns off keep-alives and disconnects every time the cache is full (will attempt
to reconnect once it run below lowwater mark)

related-to-bug: 5433309

Change-Id: Id2f942fc956e0e156834cfcd6bb08dae6a29fae1