[Issue #1266] Implementing a high-performance visibility management solution

[gengdy1545]

This PR introduces a high-performance visibility management scheme based on Epoch + COW + Lock-Free List.

Key Components:

1. EpochManager:

   • Implements an epoch-based memory reclamation mechanism.
   • Query threads declare active status via enterEpoch() and exitEpoch().
   • GC threads advance the Epoch after retiring old data, only releasing memory safely once all threads in the old Epoch have exited.

2. VersionedData & COW:

   • Encapsulates the base bitmap and deletion record list head within VersionedData.
   • GC operations create new VersionedData copies via Copy-on-Write.
   • Utilises atomic compare-and-swap (CAS) on std::atomic<VersionedData*> currentVersion to ensure query threads consistently observe a coherent snapshot.

[bianhq]

LGTM.