Cleanup bench-exchange (#3919)

* bench-exchange changes

Generate new keypair for each bench-exchange
Add metrics
Tweak network sleep parameters for better reliability.

* Bench-exchange contract metrics

bench-exchange/src/main.rs

@@ -9,6 +9,7 @@ use solana_sdk::signature::KeypairUtil;
 
 fn main() {
     solana_logger::setup();
+    solana_metrics::set_panic_hook("bench-exchange");
 
     let matches = cli::build_args().get_matches();
     let cli_config = cli::extract_args(&matches);