Add support for edges (lines) between nodes

[alexander-bauer]

This would make creating peering maps much simpler. How to add them will be a separate issue. I think they should be stored in the database as relative between unique int64 node IDs, which lower the amount of storage space required (16 bytes per IP, meaning at least 32 bytes per link vs. 8 bytes per ID, meaning only 16 bytes per link).