Multi-database support

[emi420]

This PR adds multi-database support, including the following:

• Multi-database support is optional, default configuration is single-database. The Python DB/REST API includes services for querying data from both databases.
• The Python DB/REST API was refactored for clarity and for supporting querying Statistics and Raw OSM Data using different databases. Validation queries will be added later. For querying Validation and Raw OSM Data together, a single-database configuration is required, at least for now.
• default.yaml file now includes two variables underpass_db_url and underpass_osm_db_url, same with environment variables REPLICATOR_OSM_DB_URL and REPLICATOR_UNDERPASS_DB_URL and --rawdb and --server underpass parameters.
• Docker compose file has commented code for the Raw database
• The --bootstrap command now supports multi-database