avoid reusing context object

fix https://github.com/chrislusf/seaweedfs/issues/1182

weed/shell/command_volume_copy.go

@@ -1,7 +1,6 @@
 package shell
 
 import (
-	"context"
 	"fmt"
 	"io"
 
@@ -47,7 +46,6 @@ func (c *commandVolumeCopy) Do(args []string, commandEnv *CommandEnv, writer io.
 		return fmt.Errorf("source and target volume servers are the same!")
 	}
 
-	ctx := context.Background()
-	_, err = copyVolume(ctx, commandEnv.option.GrpcDialOption, volumeId, sourceVolumeServer, targetVolumeServer)
+	_, err = copyVolume(commandEnv.option.GrpcDialOption, volumeId, sourceVolumeServer, targetVolumeServer)
 	return
 }