[avalanche] Promote and poll contenders as soon as quorum is active

Summary:
This diff accomplishes a few things:

1. Contender promotion becomes generally more robust to future changes.
2. A miner restarting a node no longer faces uncertainty of the staking rewards payout address in getblocktemplate, since prior to this change, a block would need to be mined before contenders began polling.
3. Improves resilience of the network if many nodes are restarted for any reason. This is especially important when coupled with 2.

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17986