[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