Optimize toEdge, IndexEdgeDeserializable using mutable Map.

since Storage#toEdge is called lots of times(# of edges query traverse), I think it is worth to optimize futher.

based on profile, we can notice that List#toMap and List#++ on two list takes many cpu time on toEdge(actually IndexEdgeDeserializable#fromKeyValues).

we can avoid List#toMap and List#++ by using MapBuilder and give up functional style here.

I think code looks ugly(maybe not), but we can remove unnecessary cpu usage.