Running master with `src/bitcoind -zmqpubhashblockhwm=12345 -zmqpubrawtx=tcp://127.0.0.1:28332 -zmqpubhashtxhwm=5000 -zmqpubrawblock=tcp://127.0.0.1:28332 -zmqpubhashtx=tcp://127.0.0.1:28332 -zmqpubhashblock=tcp://127.0.0.1:28332 -debug=zmq`:
```
2019-01-19T14:11:01Z zmq: version 4.3.1
2019-01-19T14:11:01Z zmq: Initialize notification interface
2019-01-19T14:11:01Z zmq: Outbound message high water mark for pubhashblock at tcp://127.0.0.1:28332 is 12345
2019-01-19T14:11:01Z zmq: Notifier pubhashblock ready (address = tcp://127.0.0.1:28332)
2019-01-19T14:11:01Z zmq: Reusing socket for address tcp://127.0.0.1:28332
2019-01-19T14:11:01Z zmq: Notifier pubhashtx ready (address = tcp://127.0.0.1:28332)
2019-01-19T14:11:01Z zmq: Reusing socket for address tcp://127.0.0.1:28332
2019-01-19T14:11:01Z zmq: Notifier pubrawblock ready (address = tcp://127.0.0.1:28332)
2019-01-19T14:11:01Z zmq: Reusing socket for address tcp://127.0.0.1:28332
2019-01-19T14:11:01Z zmq: Notifier pubrawtx ready (address = tcp://127.0.0.1:28332)
```
This PR:
```
2019-01-19T14:06:57Z zmq: version 4.3.1
2019-01-19T14:06:57Z zmq: Initialize notification interface
2019-01-19T14:06:57Z zmq: Outbound message high water mark for pubhashblock at tcp://127.0.0.1:28332 is 12345
2019-01-19T14:06:57Z zmq: Notifier pubhashblock ready (address = tcp://127.0.0.1:28332)
2019-01-19T14:06:57Z zmq: Reusing socket for address tcp://127.0.0.1:28332
2019-01-19T14:06:57Z zmq: Outbound message high water mark for pubhashtx at tcp://127.0.0.1:28332 is 5000
2019-01-19T14:06:57Z zmq: Notifier pubhashtx ready (address = tcp://127.0.0.1:28332)
2019-01-19T14:06:57Z zmq: Reusing socket for address tcp://127.0.0.1:28332
2019-01-19T14:06:57Z zmq: Outbound message high water mark for pubrawblock at tcp://127.0.0.1:28332 is 1000
2019-01-19T14:06:57Z zmq: Notifier pubrawblock ready (address = tcp://127.0.0.1:28332)
2019-01-19T14:06:57Z zmq: Reusing socket for address tcp://127.0.0.1:28332
2019-01-19T14:06:57Z zmq: Outbound message high water mark for pubrawtx at tcp://127.0.0.1:28332 is 1000
2019-01-19T14:06:57Z zmq: Notifier pubrawtx ready (address = tcp://127.0.0.1:28332)
```
Backport of Bitcoin Core PR15209
https://github.com/bitcoin/bitcoin/pull/15209