Merge #18742: miner: Avoid stack-use-after-return in validationinterface

7777f2a4bb1f9d843bc50a4e35085cfbb2808780 miner: Avoid stack-use-after-return in validationinterface (MarcoFalke)
fa5ceb25fce2200edf6b8ebfa6d4f01ed6774b95 test: Remove UninterruptibleSleep from test and replace it by SyncWithValidationInterfaceQueue (MarcoFalke)
fa770ce7fe67685c43780e219d8232efbee0bb8e validationinterface: Rework documentation, Rename pwalletIn to callbacks (MarcoFalke)
fab6d060ce5f580db538070beec1c5518c8c777c test: Add unregister_validation_interface_race test (MarcoFalke)

Pull request description:

  When a validationinterface has itself unregistered in one thread, but is about to get executed in another thread [1], there is a race:

  * The validationinterface destructing itself
  * The validationinterface getting dereferenced for execution

  [1] https://github.com/bitcoin/bitcoin/blob/64139803f1225dab26197a20314109d37fa87d5f/src/validationinterface.cpp#L82-L83

  This happens in the miner. More generally it happens everywhere where at least one thread is generating notifications and another one is unregistering a validationinterface.

  This issue has been fixed in commit ab31b9d6fe7b39713682e3f52d11238dbe042c16, but the fix has not been applied to the miner.

  Example where this happened in practice: https://travis-ci.org/github/bitcoin/bitcoin/jobs/675322230#L4414

ACKs for top commit:
  promag:
    Code review ACK 7777f2a4bb1f9d843bc50a4e35085cfbb2808780.
  laanwj:
    Code review ACK 7777f2a4bb1f9d843bc50a4e35085cfbb2808780

Tree-SHA512: 8087119243c71ba18a823a63515f3730d127162625d8729024278b447af29e2ff206f4840ee3d90bf84f93a2c5ab73b76c7e7044c83aa93b5b51047a166ec3d3

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