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

core/src/banking_stage.rs

@@ -547,7 +547,7 @@ impl BankingStage {
             timing::duration_as_ms(&recv_start.elapsed()),
             count,
         );
-        inc_new_counter_info!("banking_stage-entries_received", mms_len);
+        inc_new_counter_info!("banking_stage-transactions_received", count);
         let proc_start = Instant::now();
         let mut new_tx_count = 0;