obs-outputs: Fix RTMP restart not always working

Bug is caused by the internal connection variables not being reset on
reconnect, leading OBS to both be unable to parse valid packets from and
send valid packets to the remote end.  This commit splits RTMP_Init off
into a new RTMP_Reset function, which resets these internal variables
without re-initing the rest of the library.  The original RTMP_Init
calls the new function, perfectly preserving the old behaviour while
adding a new reset function to address the issue with.

Fixes obsproject/obs-studio#2865