Properly handle fetcher pool when exiting

Context manager does not seem to properly wait for the subprocesses to exit, let's join
the pool manually (close need to be called before joining)

pghoard/restore.py

@@ -547,6 +547,10 @@ def fetch_all(self):
                     with self.pool_class(processes=self._process_count()) as pool:
                         self._queue_jobs(pool)
                         self._wait_for_jobs_to_complete()
+                        # Context manager does not seem to properly wait for the subprocesses to exit, let's join
+                        # the pool manually (close need to be called before joining)
+                        pool.close()
+                        pool.join()
                         break
             except TimeoutError:
                 self.pending_jobs.clear()