-Add LBRYFileProducer, to host the contents of a download without
having to keep re-opening it as it is added to
-included sd hash in ManagedLBRYFileDownloader, to make comparing the
contents of the file manager against name claims easier
-add get_lbry_file function, which returns information about a LBRY
file found by sd_hash, file name, or lbry uri