[avalanche] Add support to poll multiple block at once.
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
Differential Revision: https://reviews.bitcoinabc.org/D2121