Refactor: tools: Create a new data set every time we display.

The previous behavior was where we had a global mon_data_set that would
get created once, and then reset every time mon_refresh_display was
finished. Presumably, this was for caching purposes. However, I don't
believe it was saving all that much. We still had to unpack everything
into the data set every time through.

So instead, just make mon_data_set a local variable and create/destroy
it every time through mon_refresh_display.