shutdown: terminate subprocesses

* We use a subprocess pool for running backgorund commands
  (e.g. `cylc clean`).
* This creates subprocesses to run our commands in, however, it does
  not kill the subprocess once the command has completed, it keeps
  the subprocess open for future reuse (more efficient than creating
  and destroying them every time).
* On shutdown we need to close the pool to mop up these subprocesses
  (this doesn't happen automatically).

changes.d/619.fix.md

@@ -0,0 +1 @@
+Ensure that subprocesses created by Cylc UI Server are cleaned up correctly when the server shuts down.

cylc/uiserver/app.py

@@ -580,10 +580,17 @@ def launch_instance(cls, argv=None, workflow_id=None, **kwargs):
     async def stop_extension(self):
         # stop the async scan task
         await self.workflows_mgr.stop()
+
+        # stop active subscriptions
         for sub in self.data_store_mgr.w_subs.values():
             sub.stop()
-        # Shutdown the thread pool executor
+
+        # Shutdown the thread pool executor (used for subscription processing)
         self.data_store_mgr.executor.shutdown(wait=False)
+
+        # stop the process pool (used for background commands)
+        self.executor.shutdown()
+
         # Destroy ZeroMQ context of all sockets
         self.workflows_mgr.context.destroy()
         self.profiler.stop()