async: URL scheme

[1] FFmpeg Protocols Documentation (2017-01-22 02:22:36 +09:00) <https://ffmpeg.org/ffmpeg-protocols.html#async>

Asynchronous data filling wrapper for input stream.
Fill data in a background thread, to decouple I/O operation from demux thread.
async:URL
async:http://host/resource
async:cache:http://host/resource