Implement optimization

[asg0451]

Optimize RetriedStorage methods by consolidating RocksDB operations and their retry logic into a single spawn_blocking call, reducing task churn.

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[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
_{Learn more about Cursor Agents}

[github-actions]

Criterion benchmarks delta (ms; lower is better)

Benchmark	PR (ms)	Master (ms)	Δ (ms)	Δ (%)
e2e_rpc/rpc_add_batch_200	0.690	0.625	0.065	+10.4%
e2e_rpc/rpc_poll_batch_200	47.516	45.106	2.410	+5.3%
e2e_rpc/rpc_remove_each_200	31.974	24.864	7.110	+28.6%
e2e_rpc/rpc_stress_like_p2_a2_items_200	1547.002	937.106	609.896	+65.1%
storage_add/add_available_500	8.080	7.985	0.095	+1.2%
storage_poll/get_next_available_entries_500	10.370	10.902	-0.532	-4.9%
storage_remove/remove_in_progress_500	20.369	19.037	1.332	+7.0%