structured cache: return unique_ptr from Create*Batch factoriessb/fix-batch-ptr

- ZenCacheNamespace::CreatePutBatch/CreateGetBatch now return
  std::unique_ptr so ownership is explicit at the call site
- ZenCacheNamespace::PutBatchHandle/GetBatchHandle own their disk-layer
  handle and clean it up in the destructor, so the public
  DeletePutBatch/DeleteGetBatch entry points on ZenCacheNamespace are
  removed
- ZenCacheStore::PutBatch/GetBatch store the handle as unique_ptr and
  their destructors collapse to `= default`; the try/catch wrappers are
  no longer needed since destruction is driven by the destructors of
  types that do not throw
- Disk-layer public API (CreatePutBatch, DeletePutBatch, etc.) is
  untouched because its inner batch-handle structs live in the .cpp and
  exposing them to the header to satisfy std::unique_ptr's completeness
  requirement would leak implementation details

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