stream_filter/httplive.c: Cleanup hls_thread_t and stream_sys_t structs.

Cleanup hls_thread_t and stream_sys_t structs:
- duplicate members
- rename hls_playlist_t to hls_playlist_s
- move download members to stream_sys_t inside a hls_download_s struct
- check for p_sys->thread is no longer needed in segment_Seek()