caching of timestamps for mpeg-ps so seeking is faster

caching of timestamps for mpeg-ps so seeking is faster
move (av_)find_stream_index() to utils.c as its usefull outside mpeg.c
assert checking enabled, to find bugs quicker, should obviously be disabled later
(av_)add_index_entry() inserts new entries so that the list stays ordered and updates entries if already in it
(av_)index_search_timestamp() cleanup (kill ugly goto) and shorter

Originally committed as revision 2697 to svn://svn.ffmpeg.org/ffmpeg/trunk