perf: faster allocator on sysbench settings

We're seeing some inter-run instability when running sysbench oltp_read_write
to the tune of 1-2% CoV with repeated 10 minute runs. Some digging has pointed
to the fact that the allocator is both not aggressive enough at the beginning
of runs, and that its CPU threshold for moving leaseholders is too large.
Racheting up the aggressiveness and lowering the thresholds helps improve
repeated run performance and lower the CoV. Adding these settings to the
"settings" run to see if it helps stabilize things there.

Epic: none
Release note: none

Author: ajstorm

pkg/cmd/roachtest/tests/sysbench.go

@@ -380,6 +380,8 @@ func registerSysbench(r registry.Registry) {
 						`set cluster setting sql.metrics.statement_details.enabled = false`,
 						`set cluster setting kv.split_queue.enabled = false`,
 						`set cluster setting kv.transaction.write_buffering.enabled = true`,
+						`set cluster setting kv.allocator.load_based_rebalancing_interval = '10s'`,
+						`set cluster setting kv.allocator.store_cpu_rebalance_threshold = 0.01`,
 					},
 					useDRPC: true,
 				}