In-memory store: Store edge properties only once

[dominikbraun]

At the moment, the built-in in-memory store implementation that satisfies the Store interface stores the edge properties multiple times: In outEdges and in inEdges, because both store full Edge[K] instances.

This isn't memory-efficient for edges that store a lot of data. Therefore, it might make sense to only store the actual Edge instances or at least their properties once, maybe in a separate map, and retrieve this data when returning an edge.