Rate limit UDP output

[andrewkroh]

The UDP output will dump all data as fast as possible. It will overload buffers of the recipient resulting in data loss. The output implementation should have some rate limit that's configurable (e.g. 500 KiB/sec).

There's a TODO here, but that's probably not the correct place for the logic as it would only apply to PCAP.

https://github.com/andrewkroh/stream/blob/7e151a6c712c398d433bc30cb9cd92c0d80959e6/command/pcap.go#L77