Update NATS to 2.10.20, use SyncAlways (#3418)

The internal NATS instance is definitely convenient but it does have one
problem: its lifecycle is tied to the Dendrite process. That means if
Dendrite panics or OOMs, it takes out NATS with it. I suspect this is
sometimes contributing to what people see with stuck streams, as some
operations or state might not be written to disk fully before it gets
interrupted.

Using `SyncAlways` means that NATS will effectively use `O_SYNC` and
block writes on flushes, which should improve resiliency against this
kind of failure considerably. It might affect performance a little but
shouldn't be significant.

Also updates NATS to 2.10.20 as there have been all sorts of fixes since
2.10.7, including better `SyncAlways` handling.

Signed-off-by: Neil Alexander <git@neilalexander.dev>

---------

Signed-off-by: Neil Alexander <git@neilalexander.dev>
Co-authored-by: Neil Alexander <neilalexander@users.noreply.github.com>

setup/jetstream/nats.go

@@ -56,6 +56,7 @@ func (s *NATSInstance) Prepare(process *process.ProcessContext, cfg *config.JetS
 			MaxPayload:      16 * 1024 * 1024,
 			NoSigs:          true,
 			NoLog:           cfg.NoLog,
+			SyncAlways:      true,
 		}
 		s.Server, err = natsserver.NewServer(opts)
 		if err != nil {