[avalanche] Create an event loop facility.

Summary:
Avalanche will need to pool peers on a regular basis, which require an event loop. This introduces the event loop, but it does nothing so far.

Depends on D2042

Test Plan:

make check

Added unit tests.

Reviewers: #bitcoin_abc, jasonbcox, schancel, Fabien

Reviewed By: #bitcoin_abc, jasonbcox, schancel, Fabien

Subscribers: teamcity, Fabien, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D2043