[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