cmd, process, factory, node, integrationTests: versioned header factory

[AdoAdoAdo]

This PR implements a versioned header factory

[sasurobert]

this is actually a stub

[iulianpascalau]

the code would have been a little clearer if it would have been like:

func (shf *shardHeaderFactory) Create(epoch uint32) data.HeaderHandler {
	version := shf.headerVersionHandler.GetVersion(epoch)

	switch version {
	case "2":
		return &block.HeaderV2{
			Header: &block.Header{
				Epoch:           epoch,
				SoftwareVersion: []byte(version),
			},
			ScheduledRootHash: nil,
		}
        default: 
                return &block.Header{
			Epoch:           epoch,
			SoftwareVersion: []byte(version),
		}
	}