Merge #21008: test: fix zmq test flakiness, improve speed

ef21fb7313005a8a2d4f03fb4056f1f66c1b04f0 zmq test: speedup test by whitelisting peers (immediate tx relay) (Sebastian Falbesoner)
5c6546362dce8b468268578e345c37ed515a1855 zmq test: fix flakiness by using more robust sync method (Sebastian Falbesoner)
8666033630eeaf851ec69e018bb53eb23093f4b9 zmq test: accept arbitrary sequence start number in ZMQSubscriber (Sebastian Falbesoner)
6014d6e1b5a0dda6e20c2721f0bdb7e6a63ece81 zmq test: dedup message reception handling in ZMQSubscriber (Sebastian Falbesoner)

Pull request description:

  Fixes #20934 by using the "sync up" method described in https://github.com/bitcoin/bitcoin/issues/20538#issuecomment-738791868.

  After improving robustness with this approach (commits 1-3), it turned out that there were still some fails, but those were unrelated to zmq: Out of 500 runs, 3 times `sync_mempool()` or `sync_blocks()` timed out, which can happen because the trickle relay time has no upper bound -- hence in rare cases, it takes longer than 60s. This is fixed by enabling immediate tx relay on node1 (commit 4), which as a nice side-effect also gives us a rough 2x speedup for the test.

  For further details, also see the explanations in the commit messages.

  There is no guarantee that the test is still not flaky, but it would help if potential reviewers would run the following script locally and report how many runs failed (feel free to do less than 1000 runs, as this takes quite a long if ran with `--valgrind`):
  ```
  #!/bin/sh
  OUTPUT_FILE=./zmq_results
  echo ===== repeated zmq test ===== > $OUTPUT_FILE

  for i in `seq 1000`; do
      echo ------------------------
      echo ----- test run $i -----
      echo ------------------------
      echo --- $i --- >> $OUTPUT_FILE
      ./test/functional/interface_zmq.py --valgrind
      if [ $? -ne 0 ]; then
          echo "FAILED. /o\\" >> $OUTPUT_FILE
      else
          echo "PASSED. \\o/" >> $OUTPUT_FILE
      fi
  done

  echo Failed test runs:
  grep FAILED $OUTPUT_FILE | wc -l
  ```

ACKs for top commit:
  jonatack:
    Light ACK ef21fb7313005a8a2d4f03fb4056f1f66c1b04f0 with the caveat that I was unable to make the test fail with valgrind both here and on master, so I can't vouch that it actually fixes the CI flakiness. The test does run ~2x faster with this.

Tree-SHA512: 7a1e7592fbbd98e69e1e1294486b91253e589c72b3c6bbb7f587028ec07cca59b7d984e4ebf256c4bc3e8a529ec77d31842f3dd874038aea0b684abfea50306a

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