graph/db: let GraghCache.AddChannel take CachedEdgeInfo

Define a new CachedEdgeInfo type and let the graph cache's AddChannel
use this. This will let us later on (for the SQL impl of the graph db)
only load from the DB what we actually need for the graph cache.

graph/db/graph_cache_test.go

@@ -61,7 +61,7 @@ func TestGraphCacheAddNode(t *testing.T) {
 		}
 		cache := NewGraphCache(10)
 		cache.AddNodeFeatures(nodeA, lnwire.EmptyFeatureVector())
-		cache.AddChannel(&models.ChannelEdgeInfo{
+		cache.AddChannel(&models.CachedEdgeInfo{
 			ChannelID: 1000,
 			// Those are direction independent!
 			NodeKey1Bytes: pubKey1,