feat: snapshot (non-serializable) reads in OptimisticTxDatabase

[polaz]

Upstream Reference

fjall-rs#271

Why CoordiNode Needs This

Currently ALL reads in OptimisticTxDatabase are serializable — every get() is tracked in the conflict manager. For graph traversals that are read-heavy, this causes unnecessary OCC abort storms.

CoordiNode needs the RocksDB distinction between:

• get() — snapshot read, no conflict tracking (for read-only traversals)
• get_for_update() — serializable read, tracked for conflict detection (for read-modify-write)

Without this, read-heavy graph traversals that don't modify data still cause transaction conflicts and retries.

Priority

P0 — blocks efficient read-heavy graph traversals under OCC

Status

Open upstream. Consider implementing in fork if upstream doesn't move.