After Client.close():

• all workers have to be killed, immediately if user doesn't want to wait for results (which they are now too, but in a blocking manner)
• all solvers have to be invalidated, so no future submissions are possible (channel is closed),
• all futures have to be resolved (with timeout/shutdown error if Client.close(wait=False))

EDIT: for consistency with ThreadPoolExecutor.shutdown(), we're not implementing "immediate close" option. We could support wait=False behavior, but for little benefit -- all jobs/workers would need to finish either way, the difference would only be in close() returning immediately or not. The program wouldn't finish (exit would block) until all workers finished.