Add PostgreSQL support for map - SCKAN node mappings and update CompetencyDatabase

[napakalas]

Based on AnatomicMaps/flatmap-maker#174, available node-mappings need to be persisted in PostgreSQL.

• Add a new table path_node_mappings to store mappings between map nodes and SCKAN nodes:
  • source_id (varchar, NOT NULL) — map UUID
  • path_id (varchar, NOT NULL) — neuron population ID
  • node_id (varchar, NOT NULL) — map node ID
  • sckan_id (varchar) — SCKAN source ID
  • sckan_node_id (varchar) — SCKAN node ID
• Upgrade the database schema and CompetencyDatabase.
• Update __update_connectivity and __delete_source_from_tables to store and clean up entries in path_node_mappings.