Add optional extended send buffer to NetControl

When we are currently having an internet connection that stalls every now and then for just a tiny moment, the visualization starts to stutter. That's because new messages in the monitor protocol are discarded because the previous message hasn't been sent entirely yet.

This patch allows to not discard new messages and append them to the send buffer as well. This feature is disabled by default. However, it can be activated by calling enableExtendedSendBuffer() on the MonitorControl object in simspark.rb. I chose 10 megabytes for the size of this extended send buffer. That should represent about 4 or 5 seconds of data. If that's not enough, it's probably best to discard some data anyway in order to catch up again. If this turns out to be a bad default value, we can still change it. Or we can add a method to set it from some Ruby script as well. But having this rather small implementation is probably the most practical solution for now.

I think this might be helpful for competitions with unstable internet connections.