pagebench: add batch support (#12133)

## Problem

The new gRPC page service protocol supports client-side batches. The
current libpq protocol only does best-effort server-side batching.

To compare these approaches, Pagebench should support submitting
contiguous page batches, similar to how Postgres will submit them (e.g.
with prefetches or vectored reads).

## Summary of changes

Add a `--batch-size` parameter specifying the size of contiguous page
batches. One batch counts as 1 RPS and 1 queue depth.

For the libpq protocol, a batch is submitted as individual requests and
we rely on the server to batch them for us. This will give a realistic
comparison of how these would be processed in the wild (e.g. when
Postgres sends 100 prefetch requests).

This patch also adds some basic validation of responses.

Cargo.lock

@@ -4237,6 +4237,7 @@ version = "0.1.0"
 dependencies = [
  "anyhow",
  "async-trait",
+ "bytes",
  "camino",
  "clap",
  "futures",