Snapshot session records and tokens atomically

[nightlessbaron]

Summary

• take the legacy session records and accumulated-token checkpoint under one lock
• prevent GET from returning N records with N+1 turns of tokens during the Phase-3 commit window
• copy the snapshot before response serialization

Dependency

This is stacked on #66 and temporarily includes #66 in the prod diff. Rebase onto prod after #66 lands.