Merge bitcoin/bitcoin#22577: Close minor startup race between main and scheduler threads

703b1e612a4bd4521e20ae21eb8fb7c19f4ef942 Close minor startup race between main and scheduler threads (Larry Ruane)

Pull request description:

  This is a low-priority bug fix. The scheduler thread runs `CheckForStaleTipAndEvictPeers()` every 45 seconds (EXTRA_PEER_CHECK_INTERVAL). If its first run happens before the active chain is set up (`CChain::SetTip()`), `bitcoind` will assert:
  ```
  (...)
  2021-07-28T22:16:49Z init message: Loading block index…
  bitcoind: validation.cpp:4968: CChainState& ChainstateManager::ActiveChainstate() const: Assertion `m_active_chainstate' failed.
  Aborted (core dumped)
  ```
  I ran into this while using the debugger to investigate an unrelated problem. Single-stepping through threads with a debugger can cause the relative thread execution timing to be very different than usual. I don't think any automated tests are needed for this PR. I'll give reproduction steps in the next PR comment.

ACKs for top commit:
  MarcoFalke:
    cr ACK 703b1e612a4bd4521e20ae21eb8fb7c19f4ef942
  tryphe:
    tested ACK 703b1e612a4bd4521e20ae21eb8fb7c19f4ef942
  0xB10C:
    ACK 703b1e612a4bd4521e20ae21eb8fb7c19f4ef942
  glozow:
    code review ACK 703b1e612a4bd4521e20ae21eb8fb7c19f4ef942 - it makes sense to me to start peerman's background tasks here, after `chainstate->LoadChainTip()` and `node.connman->Start()` have been called.

Tree-SHA512: 9316ad768cba3b171f62e2eb400e3790af66c47d1886d7965edb38d9710fc8c8f8e4fb38232811c9346732ce311d39f740c5c2aaf5f6ca390ddc48c51a8d633b

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