adds bitflags to Packet.Meta

Instead of a separate bool type for each flag, all the flags can be
encoded in a type-safe bitflags encoded in a single u8:
https://github.com/solana-labs/solana/blob/d6ec103be/sdk/src/packet.rs#L19-L31

core/benches/sigverify_stage.rs

@@ -54,7 +54,7 @@ fn bench_packet_discard(bencher: &mut Bencher) {
         SigVerifyStage::discard_excess_packets(&mut batches, 10_000);
         for batch in batches.iter_mut() {
             for p in batch.packets.iter_mut() {
-                p.meta.discard = false;
+                p.meta.set_discard(false);
             }
         }
     });