DEV: remove exec_sql and replace with mini_sql

Introduce new patterns for direct sql that are safe and fast.

MiniSql is not prone to memory bloat that can happen with direct PG usage.
It also has an extremely fast materializer and very a convenient API

- DB.exec(sql, *params) => runs sql returns row count
- DB.query(sql, *params) => runs sql returns usable objects (not a hash)
- DB.query_hash(sql, *params) => runs sql returns an array of hashes
- DB.query_single(sql, *params) => runs sql and returns a flat one dimensional array
- DB.build(sql) => returns a sql builder

See more at: https://github.com/discourse/mini_sql

script/import_scripts/modx.rb

@@ -236,7 +236,7 @@ FROM #{TABLE_PREFIX}discuss_users
   def not_mark_topics_as_solved
     puts "", "Marking topics as solved..."
 
-    PostAction.exec_sql <<-SQL
+    DB.exec <<~SQL
       INSERT INTO topic_custom_fields (name, value, topic_id, created_at, updated_at)
       SELECT 'accepted_answer_post_id', pcf.post_id, p.topic_id, p.created_at, p.created_at
         FROM post_custom_fields pcf
@@ -469,7 +469,7 @@ FROM #{TABLE_PREFIX}discuss_users
       WHERE id IN (SELECT topic_id FROM closed_topic_ids)
     SQL
 
-    Topic.exec_sql(sql, closed_topic_ids)
+    DB.exec(sql, closed_topic_ids)
   end
 
   def not_post_process_posts