rtmpproto: Track last received timestamp

Some RTMP commands need the most recent timestamp as their parameter, so
keep track of it. This must be the most recent one and not e.g. the max
received timestamp as it can decrease again through seeking.

Signed-off-by: Martin Storsjö <martin@martin.st>
rtmpproto: Add pause support

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