Expose map–SCKAN node mappings via flatmap-server

[napakalas]

Based on AnatomicMaps/flatmap-maker#174, node-mappings generated by map-maker should be exposed by flatmap-server.
With the availability of path_node_mappings (AnatomicMaps/map-knowledge#63), add a new SQL query to the CQ database.

• Input:
  • source_id
  • path_id
• Output:
  • sckan_id
  • path_id
  • sckan_node_id
  • sckan_node_label
  • source_id
  • node_id
  • node_label

The query should return mappings between map nodes and their corresponding SCKAN nodes for the given source and path.
Add a test query to validate correct results for a given source_id and path_id.