ipnserver and logpolicy: configure zstd with low-memory settings.

The compressed blobs we send back and forth are small and infrequent,
which doesn't justify the 8MB * GOMAXPROCS memory that was being
allocated. This was the overwhelming majority of memory use in
tailscaled. On my system it goes from ~100M RSS to ~15M RSS (which is
still suspiciously high, but we can worry about that more later).

Signed-off-by: Avery Pennarun <apenwarr@tailscale.com>

ipn/ipnserver/server.go

@@ -113,7 +113,11 @@ func Run(rctx context.Context, logf logger.Logf, logid string, opts Options, e w
 		return fmt.Errorf("NewLocalBackend: %v", err)
 	}
 	b.SetDecompressor(func() (controlclient.Decompressor, error) {
-		return zstd.NewReader(nil)
+		return zstd.NewReader(nil,
+			zstd.WithDecoderLowmem(true),
+			zstd.WithDecoderConcurrency(1),
+			zstd.WithDecoderMaxMemory(65536),
+		)
 	})
 
 	if opts.DebugMux != nil {