rtmp: Follow Flash player numbering for channels.

Channel 4 is typically used by the Flash player to transmit
audio, channel 6 for video, and various stream-specific invokes
get sent over channel 8, which is designated the source channel.

This more closely matches the behavior of the Flash player,
including the transmission of play requests over channel 8.

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