Make grpc plugin client use an atomic server value to fix a data race. (#4089)

Also add some coordination to ensure we don't try to clean up the grpc
server before it's created/started

logical/plugin/backend.go

@@ -3,6 +3,7 @@ package plugin
 import (
 	"context"
 	"net/rpc"
+	"sync/atomic"
 
 	"google.golang.org/grpc"
 
@@ -42,10 +43,17 @@ func (b BackendPlugin) GRPCServer(broker *plugin.GRPCBroker, s *grpc.Server) err
 }
 
 func (p *BackendPlugin) GRPCClient(ctx context.Context, broker *plugin.GRPCBroker, c *grpc.ClientConn) (interface{}, error) {
-	return &backendGRPCPluginClient{
+	ret := &backendGRPCPluginClient{
 		client:     pb.NewBackendClient(c),
 		clientConn: c,
 		broker:     broker,
+		cleanupCh:  make(chan struct{}),
 		doneCtx:    ctx,
-	}, nil
+	}
+
+	// Create the value and set the type
+	ret.server = new(atomic.Value)
+	ret.server.Store((*grpc.Server)(nil))
+
+	return ret, nil
 }