do not return duplicate nodes/ways/etc

[hackergrrl]

The OSM API does not permit duplicate IDs to be returned. To this end, osm-p2p-server has a forks option to allow forking data to be opted in to.

However, there is an edge case where one bounding box query might include one fork of a node, and a subsequent bbox query nearby would include another fork of that node in another position.

@gmaclennan has suggested we patch the indexing logic in osm-p2p-db such that bounding box queries that grab a node that has forks will return all forks of that node: this will enable osm-p2p-server to deduplicate accurately on incoming requests.