[avalanche] Poll tip candidate rather than eagerly

Summary:
No point in polling for block that may not end up as tip anyways. The next step is to poll all valid blocks.

Depends on D5673

Test Plan: Updated intergration test to reflect that the chain we poll must now be longer.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5678