Closing dangling stream

I'm creating temporary Clients to run custom task graphs on my remote cluster and was having problems with lots of socket connections sticking around and results stuck on the cluster due to exceptions causing the results on Futures to not be requested.

Explicitly calling Client.close() seemed to fix all that as I don't see any more stuck things on the cluster after an exceptions. But now I'm seeing a TCP connection not getting closed cleanly. Here's some code to reproduce the problem:

```
from dask.distributed import Client, as_completed
import uuid

def get_results():
c = Client('HOSTNAME:8786', set_as_default=False)

def daskfn():
return 'results'

futs = []
for i in range(100):
key = f'testfn-{uuid.uuid4()}'
futs.append(c.get({key: (daskfn, )}, key, sync=False))

results = []
try:
for f in as_completed(futs):
results.append(f.result())
finally:
c.close()
return results

results = get_results()
```

After this, using the command line program `ss`, I see a socket in CLOSE-WAIT state. And, if I do something that triggers garbage collection or call gc.collect(), I see the following warning:

```
distributed.comm.tcp - WARNING - Closing dangling stream in
```