[release-v2.0] chain: Wait for errgroup before returning from sync

If syncing over JSON-RPC is being reattempted in a loop, as the main package
does, not waiting for the errgroup that runs the wallet's mixing client may
cause it to concurrently enter mixclient.Client.Run again before it has
returned.  This eventually results in a panic due to closing the already-closed
warming chan.

Backport of 8b9a63d.

chain/sync.go

@@ -632,7 +632,14 @@ func (s *Syncer) Run(ctx context.Context) (err error) {
 		return err
 	}
 
+	ctx, cancel := context.WithCancel(ctx)
 	g, ctx := errgroup.WithContext(ctx)
+	defer func() {
+		cancel()
+		if e := g.Wait(); err == nil {
+			err = e
+		}
+	}()
 	g.Go(func() error {
 		// Run wallet background goroutines (currently, this just runs
 		// mixclient).