Optimize UDP checksumming, remove produced data double buffering

In case when only the addr seq cnt field changes (in each send event), we may also use incremental updating as described in RFC1071.

Also, the static checksums of the frame header and the pseudo header can be combined into one. The additional direct pointer to the checksum field can be removed, should be calculated intermediately.
 

Furthermore, the double buffering of the produced data can be removed if we make sure that the FrameUpdate() and the SendFrame() functions are always called in the scope of the high prio worker thread. This will save data payload size bytes for each producer created.