fix: make store-based barrier reusable across updates

[RunningLeon]

fix: make the shared-store barrier reusable across updates in #106

Summary

• add an independent generation counter for ParameterServer.store_based_barrier();
• use a unique _store_based_barrier() group name for every invocation;
• keep the long-lived root TCPStore unchanged and shared;
• add CPU-only regression coverage for two consecutive barriers on the same TCPStore.