Merge #19572: ZMQ: Create "sequence" notifier, enabling client-side mempool tracking

759d94e70f6844443106404882c7b105f3a4dba7 Update zmq notification documentation and sample consumer (Gregory Sanders)
68c3c7e1bdd00bbe7d70592a8eb39520fa3f87f1 Add functional tests for zmq sequence topic and mempool sequence logic (Gregory Sanders)
e76fc2b84d065c9d06010d0a10b316f1f9d36fb9 Add 'sequence' zmq publisher to track all block (dis)connects, mempool deltas (Gregory Sanders)
1b615e61bfc464f215a1b48e6e27d1e8fc16b2d1 zmq test: Actually make reorg occur (Gregory Sanders)

Pull request description:

  This PR creates a new ZMQ notifier that gives a "total hash history" of block (dis)connection, mempool addition/substraction, all in one pipeline. It also exposes a "mempool sequence number" to both this notifier and `getrawmempool` results, which allows the consumer to use the results together without confusion about ordering of results and without excessive `getrawmempool` polling.

  See the functional test `interfaces_zmq.py::test_mempool_sync` which shows the proposed user flow for the client-side tracking of mempool contents and confirmations.

  Inspired by https://github.com/bitcoin/bitcoin/pull/19462#issuecomment-656140421
  Alternative to https://github.com/bitcoin/bitcoin/pull/19462 due to noted deficiencies in current zmq notification streams.

  Also fixes a legacy zmq test that didn't actually trigger a reorg because of identical blocks being generated on each side of the split(oops)

ACKs for top commit:
  laanwj:
    Code review ACK 759d94e70f6844443106404882c7b105f3a4dba7

Tree-SHA512: 9daf0d7d996190f3a68ff40340a687519323d7a6c51dcb26be457fbc013217ea7b62fbd0700b74b654433d2e370704feb61e5584399290692464fcfcb72ce3b7

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