Split http byte range and "byte_offset" url parameter handling,
http byte range should send "206 partial content", "byte_offset" url
parameter should not. This confused http clients leading to retries
and disconnects.
Fix wrong url parameter names for seeking.
Rework encoder selection, do not give up immediately but
wait a short while for encoder(s) to become free. This is
necessary as many http client make multiple connections
simultanuously.
Cleanup.