[avalanche] Enable avalanche by default

Summary:
This can still be opted out if desired.

Some tests are relying on reorgs to happen which is not compatible with avalanche, so proceed to manual unparking when possible or turn on automatic unparking.

Test Plan:

ninja check-extended

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D12610