[Timely] Implement LIFO processing for STL Train

[pschafhalter]

Implements LIFO processing for STL train. Older window are removed. Keys are processed in round-robin order.

Implementation details

• stl_fit_lifo spawns a worker thread which communicates with the STL fit operator via inter-thread channels.
• Upon receiving a window, the operator inserts it into the hash map of priority queues.
• The operator then checks if the python worker is done processing a window into a model. If it is, the operator retrieves the most recent window for the next key, and sends it to the python worker. The operator also forwards the model downstream in the dataflow.