ZMQ assertion error when using pyzmq==19.0.2

@yadudoc is already working on this.

Since May 4, 2021.

I've started two endpoints on Cori, one with a LocalProvider and the other with the SlurmProvider to use the Cori batch compute nodes for function execution. In both cases, the endpoints appear to start without issues, but the one configured to use batch queues failed with the following message:

```
(funcx_v0.0.6a2_py3.8) yadunand@cori08:~/.funcx/cori_batch> cat interchange.std*
2021-03-09 15:02:53 funcx.sdk.client.FuncXClient:79 [INFO] [instance:46912593216800] Creating client of type for service "funcX"
2021-03-09 15:02:54 funcx.sdk.client.FuncXClient:79 [INFO] [instance:46912593283728] Creating client of type for service "funcX"
Assertion failed: nbytes == sizeof (dummy) (src/signaler.cpp:391)
Yadu : starting local interchange
Starting local interchange with endpoint id: d26372e5-ed4c-42e5-bb54-aaadcb917d8d
Yadu : started local interchange with ports: 54793. 54361
```

The same error popped up even after multiple restarts of the Cori_batch endpoint, while oddly, the cori_local endpoint did not have any issues.

This is a ZMQ error, and updating this pyzmq package appears to have fixed it. Another point to note here is that the latest version of pyzmq is 22.0.0 which is not supported by our current requirement string `pyzmq>=19.0.0,<20.0.0`. If we update the version string now, and test with it, using the latest for 0.0.6 release would be good.

With the updated ZMQ package, the endpoint passes all tests.