[#76] Add UNIQUE keyword

Signed-off-by: Alejandro Lopez <a.lopez@yadro.com>

netmap/selector.go

@@ -140,6 +140,9 @@ func (c *context) getSelectionBase(s netmap.Selector) []nodeAttrPair {
 	attr := s.GetAttribute()
 
 	for i := range c.netMap.nodes {
+		if c.usedNodes[c.netMap.nodes[i].hash] {
+			continue
+		}
 		if isMain || c.match(f, c.netMap.nodes[i]) {
 			if attr == "" {
 				// Default attribute is transparent identifier which is different for every node.