[avalanche] Start event loop at node startup

Summary: While this is not doign much, this will allows us to start boltin on it.

Test Plan:
Ran the extended test suite.
Ran under ubsan.

I do not have a good way to run tsan and asan locally. Having a doc on how to do so and get them green would be beneficial.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: jasonbcox, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5391