BaseXPathDocumentSubprocessor shares XPathFactory

[taojing2002]

In BaseXPathDocumentSubprocessor, there are two class fields:

private static XPathFactory xpathFactory = null;
private static XPath xpath = null;

However, XpathFactory is not thread-safe and it shouldn't be a class field. But the code is running for a long time without issues.

[mbjones]

Likely only a problem when we run more than one thread within an IndexWorker, so unlikely to be an issue in our k8s deployment. But leaving this open to triage and potentially fix in the future if it might be a problem in other deployment scenarios.