Refactor LBRYStreamProducer and add Content-Length header
Refactor LBRYStreamProducer and add Content-Length header

Also fixes producer pause/unpause behavior and adds slight delay
between sending chunks

zestyr/lbry,lbryio/lbry,zestyr/lbry,lbryio/lbry,DaveA50/lbry,zestyr/lbry,DaveA50/lbry,lbryio/lbry