Extract the event loop management from the avalanche code so it can be reused.

Summary: As per title.

Test Plan:

ninja all check-all

Using GCC, clang and ubsa, build

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5728