graph/db: let GraphCache.UpdatePolicy take a cached policy

Update the GraphCache.UpdatePolicy method to take a
`models.CachedEdgePolicy` instead of a `models.ChannelEdgePolicy`.
Doing this will allow us later on to only fetch the necessary info for
populating the CachedEdgePolicy when we are populating the cache via
UpdatePolicy.

graph/db/graph.go

@@ -566,7 +566,9 @@ func (c *ChannelGraph) UpdateEdgePolicy(edge *models.ChannelEdgePolicy,
 	}
 
 	if c.graphCache != nil {
-		c.graphCache.UpdatePolicy(edge, from, to)
+		c.graphCache.UpdatePolicy(
+			models.NewCachedPolicy(edge), from, to,
+		)
 	}
 
 	select {