Reduce Victoria Metrics default time window

Related discussion #3 (comment 2989486)

By default, every request to Victoria Metrics is set with start=-30d and end=-1d if not specified otherwise. This 30d window can contain a lot of data (possibly >1GB) that is difficult for nsapi to serve back.

We should find another way to serve these kind of request, for the moment we could settle for a smaller default time window.