Unnecessary Arc::clone per row in RowWriter benchmark

[jhorstmann]

Describe the bug

The jit benchmark calls batch.schema() for each row, internally the schema method clones an Arc. Normally cloning an Arc is not a problem, but doing it for every row adds some overhead.

To Reproduce

Running the benchmark under a profiler like perf and then checking the hotspots shows some atomic instructions on the hot path

perf record cargo bench --features row,jit --bench jit

[yjshen]

Thanks @jhorstmann 👍