[avalanche] Compute staking rewards winner for each block

Summary: Currently unused, keep a map of the winners per block so we don't have to compute it on the fly everytime we need it.

Test Plan:

ninja check-avalanche

Reviewers: #bitcoin_abc, PiRK, bytesofman

Reviewed By: #bitcoin_abc, PiRK, bytesofman

Subscribers: bytesofman, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14437