[avalanche] Add support to poll multiple block at once.

Summary:
Add support and test to poll multiple blocks at once. Block are selected making sure the block with most PoW are selected first.

Depends on D2044

Test Plan: Added test cases.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D2121