[backport#15209] zmq: log outbound message high water mark when reusing socket
[backport#15209] zmq: log outbound message high water mark when reusing socket

Summary:
zmq: log outbound message high water mark when reusing socket (fanquake)

Pull request description:

  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 Core [[https://github.com/bitcoin/bitcoin/pull/15209 | PR15209]]

Test Plan:
  ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7338

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