MPI backend hangs when setting `UNIDIST_MPI_SHARED_OBJECT_STORE_THRESHOLD` to a lower threshold

[Iaroslav Igoshev]

Tested with UNIDIST_MPI_SHARED_OBJECT_STORE_THRESHOLD=100 UNIDIST_CPUS=8 UNIDIST_MPI_SHARED_OBJECT_STORE=True mpiexec -n 1 python -m pytest unidist/test/test_task.py.