Simplify the logic for freeing reused strings in RTMP_Close

Simplify the logic for freeing reused strings in RTMP_Close

Instead of having a plethora of flags indicating the current
state (just to avoid freeing them when closing the previous
connection when reconnecting), split RTMP_Close into CloseInternal,
taking a flag saying whether this is a reconnect or a normal,
final RTMP_Close call.

This also avoids leaks of some strings, if the whole session is
closed while the authentication process is underway.