Fix(Zero): Fix how Zero snapshots and purge works #7096
Merged
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Zero snapshots were being calculated in a round about way. This got worse due to a recent bug introduction, which caused Zeros to not get snapshots if we don't purge keyCommits.
This PR make Zero leader calculate the snapshots based on the checkpoints from Alpha leaders (instead of their snapshots). It then proposes this snapshot to the Zero group, causing all members to create snapshot and purge keyCommit map at the same time.
This change is