HLSProvider / OSMF : report buffer length

OSMF use NetStream as main media playback control interface. (seek/play/pause/...)
this is not the case with JWPlayer MediaProvider.
refactor the code so that OSMF can handle HLSProvider through a NetStream API :
rename Buffer.as into HLSNetStream.as, which now extends NetStream to seamlessly expose playback interface
HLSProvider5 and HLSProvider6 are still fonctional and working fine after these changes