adding hook _db_optimize_sql() to database.inc

this hook - _db_optimize_sql($query) is target to optimize SQL statement before doing any _db_query_callback(), so all arguments will still keep in %d, %s, %%, %f and %b format. it will be useful if DB backend driver trying to optimize SQL statement for better compatibility, functionality and performance.

it current status, oracle DB driver will try to use this hook for rewrite and caching SQL. this hook is useful because oracle driver can use it for overcoming the limitation of maximum 30 characters, in naming of table, column, index, trigger, etc, by re-mapping the long name into its short form from a mapping table. on the other hand, we can also cache up the mapped result, and reuse it afterward for performance boost. for example, in database.oracle.inc:

/**
 * Optimize SQL query according to different database backend
 *
 * @param $query
 *   A string containing an SQL query.
 *
 * @return
 *   A string containing an optimized SQL query.
 */
function _db_optimize_sql($query) {
  // Find out optimized SQL query in cache, return it if exists
  if ($mapped = _db_get_cached_sql($query)) {
    return $mapped;
  }

  // Clean target SQL query according to Oracle limitation
  $mapped = _db_oracle_clean_sql($query);

  // Cache the optimized SQL query
  _db_set_cached_sql($query, $mapped);

  return $mapped;
}