Performance tweaks (#4340)

* Use Rc to prevent clone of Packets

* Fix min => max in banking_stage threads.

Coalesce packet buffers better since a larger batch will
be faster through banking and sigverify.

Deconstruct batches into banking_stage from sigverify since
sigverify likes to accumulate batches but then a single banking_stage
thread will be stuck with a large batch. Maximize parallelism by
creating more chunks of work for banking_stage.

bench-streamer/src/main.rs

@@ -20,13 +20,13 @@ fn producer(addr: &SocketAddr, exit: Arc<AtomicBool>) -> JoinHandle<()> {
         w.meta.size = PACKET_DATA_SIZE;
         w.meta.set_addr(&addr);
     }
-    let msgs_ = msgs.clone();
+    let msgs = Arc::new(msgs);
     spawn(move || loop {
         if exit.load(Ordering::Relaxed) {
             return;
         }
         let mut num = 0;
-        for p in &msgs_.packets {
+        for p in &msgs.packets {
             let a = p.meta.addr();
             assert!(p.meta.size < BLOB_SIZE);
             send.send_to(&p.data[..p.meta.size], &a).unwrap();