Merge bitcoin/bitcoin#29640: Fix tiebreak when loading blocks from disk (and add tests for comparing chain ties)

0465574c12 test: Fixes send_blocks_and_test docs (Sergi Delgado Segura)
09c95f21e7 test: Adds block tiebreak over restarts tests (Sergi Delgado Segura)
18524b072e Make nSequenceId init value constants (Sergi Delgado Segura)
8b91883a23 Set the same best tip on restart if two candidates have the same work (Sergi Delgado Segura)
5370bed21e test: add functional test for complex reorgs (Pieter Wuille)
ab145cb3b4 Updates CBlockIndexWorkComparator outdated comment (Sergi Delgado Segura)

Pull request description:

  This PR grabs some interesting bits from https://github.com/bitcoin/bitcoin/pull/29284 and fixes some edge cases in how block tiebreaks are dealt with.

  ## Regarding #29284

  The main functionality from the PR was dropped given it was not an issue anymore, however, reviewers pointed out some comments were outdated https://github.com/bitcoin/bitcoin/pull/29284#discussion_r1522023578 (which to my understanding may have led to thinking that there was still an issue) it also added test coverage for the aforementioned case which was already passing on master and is useful to keep.

  ## New functionality

  While reviewing the superseded PR, it was noticed that blocks that are loaded from disk may face a similar issue (check https://github.com/bitcoin/bitcoin/pull/29284#issuecomment-1994317785 for more context).

  The issue comes from how tiebreaks for equal work blocks are handled: if two blocks have the same amount of work, the one that is activatable first wins, that is, the one for which we have all its data (and all of its ancestors'). The variable that keeps track of this, within `CBlockIndex` is `nSequenceId`, which is not persisted over restarts. This means that when a node is restarted, all blocks loaded from disk are defaulted the same `nSequenceId`: 0.
  Now, when trying to decide what chain is best on loading blocks from disk, the previous tiebreaker rule is not decisive anymore, so the `CBlockIndexWorkComparator` has to default to its last rule: whatever block is loaded first (has a smaller memory address).

  This means that if multiple same work tip candidates were available before restarting the node, it could be the case that the selected chain tip after restarting does not match the one before.

  Therefore, the way `nSequenceId` is initialized is changed to:

  - 0 for blocks that belong to the previously known best chain
  - 1 to all other blocks loaded from disk

ACKs for top commit:
  sipa:
    utACK 0465574c12
  TheCharlatan:
    ACK 0465574c12
  furszy:
    Tested ACK 0465574c12.

Tree-SHA512: 161da814da03ce10c34d27d79a315460a9c98d019b85ee35bc5daa991ed3b6a2e69a829e421fc70d093a83cf7a2e403763041e594df39ed1991445e54c16532a

test/functional/test_framework/p2p.py

@@ -866,8 +866,8 @@ class P2PDataStore(P2PInterface):
          - the on_getheaders handler will ensure that any getheaders are responded to
          - if force_send is False: wait for getdata for each of the blocks. The on_getdata handler will
            ensure that any getdata messages are responded to. Otherwise send the full block unsolicited.
-         - if success is True: assert that the node's tip advances to the most recent block
-         - if success is False: assert that the node's tip doesn't advance
+         - if success is True: assert that the node's tip is the last block in blocks at the end of the operation.
+         - if success is False: assert that the node's tip isn't the last block in blocks at the end of the operation
          - if reject_reason is set: assert that the correct reject message is logged"""
 
         with p2p_lock: