avformat/udp: Add a delay between packets for streaming to clients with short buffer

avformat/udp: Add a delay between packets for streaming to clients with short buffer

This commit enables sending UDP packets in a background thread with specified delay.
When sending packets without a delay some devices with small RX buffer
( MAG200 STB, for example) will drop tail packets in bursts causing
decoding errors.

To use it specify "fifo_size" with "packet_gap" .

The output url will looks like udp://xxx:yyy?fifo_size=<output fifo
size>&packet_gap=<delay in usecs>

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>