add support to let rtmp could keep on pushing even flv file come to the end

Your load test tool is very useful. And I would like to let it have the ability to keep on pushing rtmp when the file reader come to the end of the file and could roll back again.
First thing is modify the onmetadata head, change the stream duration to -1; second thing is using a global timestamp to add up the sum up timestamp. To make this work quick and simple, I don't use your librtmp which is stable but much complicated for me.
I 've tested with you avatar sample file, it could play for at least 1 hour(I close it manually after then) without re-connection.