time-to-index tree node
access time
the index of the corresponding cache item or a value of at least length if realtime is disabled and the access time is less than the access time in the node.
Out: If realtime is enabled, the returned index is less than length.
Obtains the index of the cache item that corresponds to node and updates the access time. If realtime is enabled, access_time is expected to be equal to or greater than the time stored in node. If disabled and the access time is less, the node will not be updated and a value of at least length returned.