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

lib/migration/base_dropper.rb

@@ -48,7 +48,7 @@ module Migration
                   "Discourse: #{column_name} in #{table_name} is readonly" :
                   "Discourse: #{table_name} is read only"
 
-      ActiveRecord::Base.exec_sql <<~SQL
+      DB.exec <<~SQL
         CREATE OR REPLACE FUNCTION #{readonly_function_name(table_name, column_name)} RETURNS trigger AS $rcr$
           BEGIN
             RAISE EXCEPTION '#{message}';

lib/migration/column_dropper.rb

@@ -12,7 +12,7 @@ module Migration
     def self.mark_readonly(table_name, column_name)
       create_readonly_function(table_name, column_name)
 
-      ActiveRecord::Base.exec_sql <<~SQL
+      DB.exec <<~SQL
         CREATE TRIGGER #{readonly_trigger_name(table_name, column_name)}
         BEFORE INSERT OR UPDATE OF #{column_name}
         ON #{table_name}
@@ -51,13 +51,13 @@ module Migration
 
     def execute_drop!
       @columns.each do |column|
-        ActiveRecord::Base.exec_sql <<~SQL
+        DB.exec <<~SQL
           DROP TRIGGER IF EXISTS #{BaseDropper.readonly_trigger_name(@table, column)} ON #{@table};
           DROP FUNCTION IF EXISTS #{BaseDropper.readonly_function_name(@table, column)} CASCADE;
         SQL
 
         # safe cause it is protected on method entry, can not be passed in params
-        ActiveRecord::Base.exec_sql("ALTER TABLE #{@table} DROP COLUMN IF EXISTS #{column}")
+        DB.exec("ALTER TABLE #{@table} DROP COLUMN IF EXISTS #{column}")
       end
     end
   end

lib/migration/table_dropper.rb

@@ -18,7 +18,7 @@ module Migration
     def self.read_only_table(table_name)
       create_readonly_function(table_name)
 
-      ActiveRecord::Base.exec_sql <<~SQL
+      DB.exec <<~SQL
         CREATE TRIGGER #{readonly_trigger_name(table_name)}
         BEFORE INSERT OR UPDATE OR DELETE OR TRUNCATE
         ON #{table_name}
@@ -37,7 +37,7 @@ module Migration
     end
 
     def droppable?
-      builder = SqlBuilder.new(<<~SQL)
+      builder = DB.build(<<~SQL)
         SELECT 1
         FROM INFORMATION_SCHEMA.TABLES
         /*where*/
@@ -52,7 +52,7 @@ module Migration
         .exec(old_name: @old_name,
               new_name: @new_name,
               delay: "#{@delay} seconds",
-              after_migration: @after_migration).to_a.length > 0
+              after_migration: @after_migration) > 0
     end
 
     def table_exists(table_name_placeholder)
@@ -67,9 +67,9 @@ module Migration
     end
 
     def execute_drop!
-      ActiveRecord::Base.exec_sql("DROP TABLE IF EXISTS #{@old_name}")
+      DB.exec("DROP TABLE IF EXISTS #{@old_name}")
 
-      ActiveRecord::Base.exec_sql <<~SQL
+      DB.exec <<~SQL
         DROP FUNCTION IF EXISTS #{BaseDropper.readonly_function_name(@old_name)} CASCADE;
       SQL
     end