JFG-767 Create package for CSV export. Implement exporting of plots

+ create download servlet
+ plot representation know what metric node it represented

https://docs.google.com/a/griddynamics.com/drawings/d/1pJEWCmi684jQbpQHq8IDQ2ZjPcQoK2lZtpt9yab6Pi4/edit
This stream automatically closed after file generating in `PlotToCsvGenerator.generateCsvFile` method. but it will be refactored little bit in next commit.