Add some optimizing to ThinClient (#4112)

Can create a multi-socketed ThinClient which will use request time
from get_recent_blockhash to tune for the best node to talk to.

client/src/perf_utils.rs

@@ -36,7 +36,18 @@ pub fn sample_txs<T>(
         total_elapsed = start_time.elapsed();
         let elapsed = now.elapsed();
         now = Instant::now();
-        let mut txs = client.get_transaction_count().expect("transaction count");
+        let mut txs;
+        match client.get_transaction_count() {
+            Err(e) => {
+                // ThinClient with multiple options should pick a better one now.
+                info!("Couldn't get transaction count {:?}", e);
+                sleep(Duration::from_secs(sample_period));
+                continue;
+            }
+            Ok(tx_count) => {
+                txs = tx_count;
+            }
+        }
 
         if txs < last_txs {
             info!("Expected txs({}) >= last_txs({})", txs, last_txs);