Merge #16112: util: Log early messages

faa2a47cd7 logging: Add threadsafety comments (MarcoFalke)
0b282f9b00 Log early messages with -printtoconsole (Anthony Towns)
412987430c Replace OpenDebugLog() with StartLogging() (Anthony Towns)

Pull request description:

  Early log messages are dropped on the floor and they'd never make it to the console or debug log. This can be tested by running the test included in this pull request without re-compiling the `bitcoind`.

  Fix that by buffering early messages and flushing them as soon as all logging options have been initialized and logging has been started.

  This pull request is identical to  "Log early messages with -printtoconsole" (#13088)  by **ajtowns**, with the following changes:
  * Rebased
  * Added docstrings for `m_buffering` and `StartLogging`
  * Switch `CCriticalSection` (aka `RecursiveMutex`) to just `Mutex` in the last commit
  * Added tests

  Fixes #16098
  Fixes #13157
  Closes #13088

ACKs for commit faa2a4:
  ajtowns:
    utACK faa2a47cd7bcdbd187035c76f8dbd0442f6818dc
  hebasto:
    ACK faa2a47cd7bcdbd187035c76f8dbd0442f6818dc
  kristapsk:
    ACK faa2a47cd7bcdbd187035c76f8dbd0442f6818dc (ran added functional test before / after recompiling, didn't do additional testing)

Tree-SHA512: 685e2882642fe2a43ce171d42862582dadb840d03cda8236a994322c389ca2a1f3f431b179b2726c155c61793543bb340c568a5455d97f8b83bc7d307a85d387

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