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/mini_sql_multisite_connection.rb

@@ -0,0 +1,38 @@
+class MiniSqlMultisiteConnection < MiniSql::Connection
+
+  class CustomBuilder < MiniSql::Builder
+
+    def initialize(connection, sql)
+      super
+    end
+
+    def secure_category(secure_category_ids, category_alias = 'c')
+      if secure_category_ids.present?
+        where("NOT COALESCE(" << category_alias << ".read_restricted, false) OR " << category_alias << ".id in (:secure_category_ids)", secure_category_ids: secure_category_ids)
+      else
+        where("NOT COALESCE(" << category_alias << ".read_restricted, false)")
+      end
+      self
+    end
+  end
+
+  class ParamEncoder
+    def encode(*sql_array)
+      # use active record to avoid any discrepencies
+      ActiveRecord::Base.send(:sanitize_sql_array, sql_array)
+    end
+  end
+
+  def self.instance
+    new(nil, param_encoder: ParamEncoder.new, type_map: self.type_map(ActiveRecord::Base.connection.raw_connection))
+  end
+  # we need a tiny adapter here so we always run against the
+  # correct multisite connection
+  def raw_connection
+    ActiveRecord::Base.connection.raw_connection
+  end
+
+  def build(sql)
+    CustomBuilder.new(self, sql)
+  end
+end