Add validate_doc_update handling to query server.

Allow validate_doc_update in a design document, by handling it when the
apropriate "language" key is set in the design document.

Example:
	{ "_id": "_design/test",
	  "_rev": "1-....",
	  "language": "python",
	  "validate_doc_update": "def f(newDoc, oDoc, userCtx):
		if 'prop' in newDoc:
			return 1
		else:
			return {'forbidden': 'Required property
			missing'}",
	/* ... */
	}