PHP-1226: Add support for the new listIndexes command in 2.7.5

Some of this is also in #741 , which should be merged first.
From reading the code, `php_mongocursor_is_valid()` only checks if `cursor->current` is not null. Isn't it possible that `php_mongocursor_advance()` might return on a failure condition before calling `php_mongocursor_load_current_element()`, which resets `cursor->current` to null and attempts to re-populate it with the next result?