Hang after map_sync is called in loop for a number of iterations

The following script hangs after about ~180 iterations on my machine
Number of iterations it gets through seems to be dependent on the time elapsed (hence the sleep)
```
import numpy as np
import ipyparallel as ipp

c = ipp.Client()
dv = c[:]
dv.execute("import time")
dv.execute("import numpy as np")

v = c.load_balanced_view()

def fun(state):
time.sleep(0.5)
return np.random.normal()

# Generation loop
for it in range(1000):
inputs = range(12)
#v = c.load_balanced_view()
outputs = np.array(v.map_sync(fun, inputs))
print "Iteration", it
```
Windows 10, Anaconda installation
`ipyparallel 6.2.4 py27_0`

Cluster is started via `ipcluster start -n 4` on powershell
Tried and same problem observed with both python 2.7 and python 3.7
Tried both load balanced and direct views
No error messages and the powershell window for ipcluster becomes unresponsive

Start by running the issue's Python reproducer with `ipcluster start -n 4` on Windows, comparing the load-balanced and direct-view cases. Trace where `map_sync` stops responding after repeated iterations; done means the loop completes without hanging and the fix is verified for both reported Python versions.

Escrita pelo modelo de indexação a partir do texto da issue.