[avalanche] Add some safety to AvalancheBlockUpdate using static asserts

Summary: AvalancheBlockUpdate rely on the two lowers bits of a CBlockIndex* to be zero to work properly. While we know this is true, enforcing this is the case using a static assert is preferable.

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox, Fabien

Reviewed By: #bitcoin_abc, jasonbcox, Fabien

Subscribers: Fabien, jasonbcox, teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2222