make DB helper functions free functions

Also changes the drop function so that it can drop multiple tables
at once.

src/nominatim_db/clicmd/refresh.py

@@ -13,7 +13,7 @@ import logging
 from pathlib import Path
 
 from ..config import Configuration
-from ..db.connection import connect
+from ..db.connection import connect, table_exists
 from ..tokenizer.base import AbstractTokenizer
 from .args import NominatimArgs
 
@@ -124,7 +124,7 @@ class UpdateRefresh:
             with connect(args.config.get_libpq_dsn()) as conn:
                 # If the table did not exist before, then the importance code
                 # needs to be enabled.
-                if not conn.table_exists('secondary_importance'):
+                if not table_exists(conn, 'secondary_importance'):
                     args.functions = True
 
             LOG.warning('Import secondary importance raster data from %s', args.project_dir)