@Override
    public void endProcessing() {
        try {
            if (poolItem != null) {
                xsltPool.returnObject(poolItem, usePool);
                poolItem = null;
            }
        } finally {
            super.endProcessing();
        }
    }
We only need a single transformer factory here as it actually doesn't do
much internally when creating a transformer handler.
stroom-pipeline/src/main/java/stroom/pipeline/filter/XsltFilter.java