Update htl_app_rtmp_publish.cpp

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 roll back.
Which 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.