When link graph jobs are cleared due to abandoning the game or exiting,
flag the job as aborted.
The link graph job running in a separate thread checks the aborted flag
periodically and terminates processing early if set.
This reduces the delay at game abandon or exit if a long-running job
would otherwise still be running.
MCF Dijkstra iterations are executed for all source nodes in a round-robin order.
Source nodes typically require different numbers of MCF Dijkstra iterations
to satisfy all of their demand.
This change is to avoid performing MCF Dijkstra iterations on source nodes which
have already been fully satisfied.
