rtsp: Resend new keepalive commands if they used stale auth

These commands are sent asynchronously, not waiting for the reply.
This reply is parsed later by ff_rtsp_tcp_read_packet or
udp_read_packet. If the reply indicates that we used stale
authentication and need to use a new nonce, resend a new keepalive
command immediately.

This is the only request sent asynchronously, so currently there's
no other command that needs to be resent in the same way.

Signed-off-by: Martin Storsjö <martin@martin.st>