Add "no load" mode

[AnnaShaleva]

Is your feature request related to a problem? Please describe.

Sometimes I need to run network without load. I need this from time to time to test some code and to check the compatibility with NeoC# or for some other purposes. I made local modifications to disable load.

Describe the solution you'd like

Add a special "no load" mode to be able to run specified network without load.

[roman-khimov]

It's somewhat strange we have this setup functionality here and not in neo-go repository. Maybe network setup can be moved out and then #92 would handle any setup?

I'm OK with a quick fix for this though, just to have something for now.