[avalanche] delay executing avalanche event loop until IBD is done
[avalanche] delay executing avalanche event loop until IBD is done

Summary: as title

Test Plan:
  cmake -DCMAKE_BUILD_TYPE=Debug; ninja all check check-functional
  build-configurations.py build-tsan

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D9371

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