Facility for Grammar Reuse (e.g. Xerces GrammarPool)

@cowtowncoder I am trying to switch eXist-db from Xerces to Woodstox (with the SAX API).

Xerces has the facility to setup a shared Grammar Pool, by setting the property `http://apache.org/xml/properties/internal/grammar-pool` and providing an `org.apache.xerces.xni.grammars.XMLGrammarPool` so that parsed Grammars (DTD, XML Schema etc) can be reused without the overhead of re-parsing them. I was wondering if Woodstox (or its use of MSV) had anything similar, or if it was even desirable to have that in Woodstox?

If it is missing but desirable, and you let me know how you envisage it being added, I am happy to do the work and send a PR...