fe7180c [trivial,doc] Fix memory consistency model in comment (Jesse Cohen)

Pull request description:

  Updating a comment overlooked during review in #13247

Tree-SHA512: 0bd54ba1c265fdd77fd6e12ad0be46dd422348f7d926ce9abaca53fdb3a3c55c0d1cd90b4382321352076f4a81e2249c0014cd789f47a3637cb93bd983cb4657

Summary:
Updating a comment overlooked during review in #13247

Backport of Bitcoin Core PR13835
bitcoin/bitcoin#13835

Test Plan:
```
make check-all
```

Reviewers: Fabien, #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4068

…mment

fe7180c [trivial,doc] Fix memory consistency model in comment (Jesse Cohen)

Pull request description:

  Updating a comment overlooked during review in bitcoin#13247

Tree-SHA512: 0bd54ba1c265fdd77fd6e12ad0be46dd422348f7d926ce9abaca53fdb3a3c55c0d1cd90b4382321352076f4a81e2249c0014cd789f47a3637cb93bd983cb4657

…mment

fe7180c [trivial,doc] Fix memory consistency model in comment (Jesse Cohen)

Pull request description:

  Updating a comment overlooked during review in bitcoin#13247

Tree-SHA512: 0bd54ba1c265fdd77fd6e12ad0be46dd422348f7d926ce9abaca53fdb3a3c55c0d1cd90b4382321352076f4a81e2249c0014cd789f47a3637cb93bd983cb4657

…mment

fe7180c [trivial,doc] Fix memory consistency model in comment (Jesse Cohen)

Pull request description:

  Updating a comment overlooked during review in bitcoin#13247

Tree-SHA512: 0bd54ba1c265fdd77fd6e12ad0be46dd422348f7d926ce9abaca53fdb3a3c55c0d1cd90b4382321352076f4a81e2249c0014cd789f47a3637cb93bd983cb4657

…tChain + optimization

50dbec5 Add unit tests for signals generated by ProcessNewBlock() (furszy)
f68251d Validation: rename one of the two instances using "bad-prevblk" to its correct description of "prevblk-not-found" (furszy)
a51a755 Fix concurrency-related bugs in ActivateBestChain (Jesse Cohen)
198f435 Do not unlock cs_main in ABC unless we've actually made progress. (Matt Corallo)
8d15cf5 Optimize ActivateBestChain for long chains (Pieter Wuille)
8640be1 Fix fast-shutdown crash if genesis block was not loaded (Matt Corallo)
ef24337 Hold cs_main while calling UpdatedBlockTip() and ui.NotifyBlockTip (Jesse Cohen)
f9d2ab3 Update ValidationInterface() documentation to explicitly specify threading and memory model (Jesse Cohen)
cb9bb25 Update documentation for SingleThreadedSchedulerClient() to specify the memory model (Jesse Cohen)
4ea2048 Add Unit Test for SingleThreadedSchedulerClient (Jesse Cohen)

Pull request description:

  Made some back ports and adaptations to validate further the work introduced in #2203 and #2209.
  Now that the scheduler thread is processing most of the chain events, the callbacks execution order must be verified to dispatch the events serially, ensuring a consistent memory state in each event processing invocation.
  There are some concurrency issues solved, as well a an optimization for larger chains for the ABC workflow. Each commit details them well.

  List:
  bitcoin#7917 (only fb8fad1)
  bitcoin#12988
  bitcoin#13023
  bitcoin#13247
  bitcoin#13835

  This is built on top of #2284 (and the reason that made me notice that problem). So, 2284 comes first, then this one.

ACKs for top commit:
  Fuzzbawls:
    ACK 50dbec5
  random-zebra:
    ACK 50dbec5 and merging...

Tree-SHA512: 573144cc96d2caa49ed2f0887dc8c53b5aca0efd54b6a25626063ccb780da426f56b3c6b7491648e7abefc1960c82b1354a4ff2c51425bfe99adaa4394562608