Semaphore gets released too often

To avoid generating too much NFS traffic, I am using a semaphore like this to copy data from NFS to a local file storage to read from later:

```
cluster = SLURMCluster(cores=16, memory="64 GB", #processes=1,
local_directory="/var/scratch/me/dask_scheduler_spill",
interface='ib0', walltime='24:00:00')

# Create a client to submit to.
client = Client(cluster)

# Allocate 10 nodes in the cluster
cluster.scale(10)

print("Waiting for workers")
# Wait until they are ready
client.wait_for_workers(10)
print("Workers are ready!")

sem = Semaphore(max_leases=4, name='data_copy')

def copy_data(location, target, sem):
with sem:
copy_tree(location, target, update=True)

client.run(copy_data, root_dataset, local_dataset, sem)
sem.close() # Clean up the semaphore at the scheduler side
```

Now if I run it I get the error that
```
site-packages/distributed/semaphore.py", line 472, in release
raise RuntimeError("Released too often")
RuntimeError: Released too often
```

I tried debugging, I found out that setting `processes=1,` in the construction of SLURMCluster does **not ** fix this. So it looks like a threading issue to me. https://github.com/dask/distributed/issues/4057 should've fixed that, but clearly it didn't? Creating the semaphore with `sem = Semaphore(max_leases=4, name='data_copy', client=client, register=True)` also did not work.