[task/config-class] Implemented a config class to replace the global array.

There is a phpbb_config class which simply holds an array and does not persist
any data. It implements ArrayAccess, Countable and IteratorAggregate to allow
regular use of configuration as if it was still an array. The phpbb_config_db
class depends on an instance of the dbal and a cache driver. It obtains the
configuration data from cache and database as necessary and persists data to
the database.

The functions set_config and set_config_count remain for backward compatability
but they only call methods on the new config class now instead of directly
manipulating the database and cache.

PHPBB3-9988

phpBB/includes/db/dbal.php

@@ -447,6 +447,28 @@ class dbal
 		return $column_name . ' | ' . (1 << $bit) . (($compare) ? ' ' . $compare : '');
 	}
 
+	/**
+	* Returns SQL string to cast a string expression to an int.
+	*
+	* @param  string $expression An expression evaluating to string
+	* @return string             Expression returning an int
+	*/
+	function cast_expr_to_bigint($expression)
+	{
+		return $expression;
+	}
+
+	/**
+	* Returns SQL string to cast an integer expression to a string.
+	*
+	* @param  string $expression An expression evaluating to int
+	* @return string             Expression returning a string
+	*/
+	function cast_expr_to_string($expression)
+	{
+		return $expression;
+	}
+
 	/**
 	* Run more than one insert statement.
 	*

phpBB/includes/db/firebird.php

@@ -465,6 +465,22 @@ class dbal_firebird extends dbal
 		return 'BIN_OR(' . $column_name . ', ' . (1 << $bit) . ')' . (($compare) ? ' ' . $compare : '');
 	}
 
+	/**
+	* @inheritdoc
+	*/
+	function cast_expr_to_bigint($expression)
+	{
+		return 'CAST(' . $expression . ' as DECIMAL(255, 0))';
+	}
+
+	/**
+	* @inheritdoc
+	*/
+	function cast_expr_to_string($expression)
+	{
+		return 'CAST(' . $expression . ' as VARCHAR(255))';
+	}
+
 	/**
 	* return sql error array
 	* @access private

phpBB/includes/db/postgres.php

@@ -365,6 +365,22 @@ class dbal_postgres extends dbal
 		return $expression;
 	}
 
+	/**
+	* @inheritdoc
+	*/
+	function cast_expr_to_bigint($expression)
+	{
+		return 'CAST(' . $expression . ' as DECIMAL(255, 0))';
+	}
+
+	/**
+	* @inheritdoc
+	*/
+	function cast_expr_to_string($expression)
+	{
+		return 'CAST(' . $expression . ' as VARCHAR(255))';
+	}
+
 	/**
 	* return sql error array
 	* @access private