Push the creation of date managing SQL into the database driver

The DatabaseConnection class has added several new methods and abstract methods:

abstract public function hasTimezoneSupport();

abstract public function dateFieldSql($field, $date_type = DATE_UNIX);

abstract public function dateMathSql($field, $direction, $count, $granularity);

abstract public function dateConvertTimezoneSql($field, $source, $target);

/**
 * Format a date field.
 *
 * @param string $format
 *   A format string for the result, like 'Y-m-d H:i:s' .
 * @param string $field
 *   The real table and field name, like 'tablename.fieldname' .
 *
 * @return string
 *   An appropriate SQL string for the db type and field type.
 */
abstract public function dateFormatSql($field, $format);

/**
 * Extract part of a date from a date field.
 *
 * @param string $extract_type
 *   The type of value to extract from the date, like 'MONTH'.
 * @param string $field
 *   The real table and field name, like 'tablename.fieldname'.
 *
 * @return string
 *   An appropriate SQL string for the db type and field type.
 */
abstract public function dateExtractSql($field, $extract_type);

Note: The original 1.8.0 version had different version signatures. They were updated to the signatures above in 1.8.1.