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

entry/src/entry.rs

@@ -549,7 +549,7 @@ pub fn start_verify_transactions(
                 );
                 let verified = packet_batches
                     .iter()
-                    .all(|batch| batch.packets.iter().all(|p| !p.meta.discard));
+                    .all(|batch| batch.packets.iter().all(|p| !p.meta.discard()));
                 verify_time.stop();
                 (verified, verify_time.as_us())
             });