add ImportBlocks

[erikzhang]

@vncoelho You can add MaxImportHeight logic to this plugin.

After that, we can remove the import block function from neo-cli and neo-gui.

[vncoelho]

Nice idea, @erikzhang. Great insight. It will proportionate more use cases and better portability.
I learned several small things on this plugin journey.

How should be done? After being merged we edit on top?
I think that it would be:

• the settings.cs file with:
  • public uint MaxOnImportHeight { get; }
  • and this.MaxOnImportHeight = (uint)GetValueOrDefault(section.GetSection("MaxOnImportHeight"), 0, p => int.Parse(p));
  • The function below inside the ImportBlocks.cs and, probably, being called at GetBlocks:

         public bool CheckMaxOnImportHeight(uint currentImportBlockIndex)
        {
	    if(Settings.Default.MaxOnImportHeight != 0 && Settings.Default.MaxOnImportHeight == currentImportBlockIndex)
		return true;
	    return false;
        }

• an additional config.json file with: "MaxOnImportHeight": 0