Flags an offset value to be written to the persistent cache.
If a value is assigned to a cache object with offsetSet(), by default it will not be written to the persistent cache unless it is flagged with this method. This allows items to be cached for the duration of a request, without necessarily writing back to the persistent cache at the end.
$offset: The array offset that was request.
$persist: Optional boolean to specify whether the offset should be persisted or not, defaults to TRUE. When called with $persist = FALSE the offset will be unflagged so that it will not written at the end of the request.
protected function persist($offset, $persist = TRUE) {
$this->keysToPersist[$offset] = $persist;
}