zmq failure behavior

```
import time
import zmq
from tqdm import trange
import json
```

Server is REQ. Client is REP.
```
# create two connecting clients
context = zmq.Context()
socket = context.socket(zmq.REP)
socket.connect("tcp://localhost:5555")

count = 0
while True:
    message = socket.recv()
    socket.send(b"")
    count += 1
    print(json.loads(message.decode())['n'])

print(count)
```

```
# create one binding server
context = zmq.Context()
socket = context.socket(zmq.REQ)
socket.bind("tcp://*:5555")

for n in trange(100):
    message = json.dumps({"n": n}).encode()
    socket.send(message)
    socket.recv()
    time.sleep(1)
```

Output:
```
$$ python responder.py
2
4
6
8
10
12
<<< fall back full on this responder
14
15
16
17
18
19

$$ python responder.py
0
1
3
5
7
9
11
13
^CTraceback (most recent call last):
  File "responder.py", line 17, in <module>
    message = socket.recv()
  File "zmq/backend/cython/socket.pyx", line 791, in zmq.backend.cython.socket.Socket.recv
  File "zmq/backend/cython/socket.pyx", line 827, in zmq.backend.cython.socket.Socket.recv
  File "zmq/backend/cython/socket.pyx", line 186, in zmq.backend.cython.socket._recv_copy
  File "zmq/backend/cython/checkrc.pxd", line 13, in zmq.backend.cython.checkrc._check_rc
KeyboardInterrupt
```

Server is REP. Client is REQ

```
# create two connecting clients
context = zmq.Context()
socket = context.socket(zmq.REP)
socket.bind("tcp://*:5555")

count = 0
while True:
    message = socket.recv()
    socket.send(b"")
    count += 1
    print(json.loads(message.decode())['n'])

print(count)
```

```
# create one binding server
context = zmq.Context()
socket = context.socket(zmq.REQ)
socket.connect("tcp://localhost:5555")
socket.connect("tcp://localhost2:5555")

for n in trange(100):
    message = json.dumps({"n": n}).encode()
    socket.send(message)
    socket.recv()
    time.sleep(1)
```


Output:
```
$$ python responder.py
0
<<< stuck
```