[rtmp] fix crash on seek on live streams

this fixes issues raised in http://forum.kodi.tv/showthread.php?tid=187840&page=2

This PR does 2 things:
1. implement CanSeek() for RTMP 
2. removes unused protocol options struct, those are parsed from the URL and handled directly in librtmp.
i would expect that the stream can't be paused either in this case?