Implement enhanced diffs

[xivk]

Implement diffs that only store changes. For example if a node position doesn't change we don't store the lat/lon again. The same for its tags or a relation's members or a node's ways.

• Node:
  • Tags
  • Lat/Lon
• Way:
  • Nodes
  • Tags.
• Relation:
  • Members
  • Tags.

Reasoning:

In a lot of cases only tags will change or only positions. When splitting a way only the nodes set changes. This will reduce diskspace used for a diff even further.