GEODE-6914: Replace libxml with xerces-c in CacheXmlParser (#501)

GEODE-6914: Replace libxml with xerces-c in CacheXmlParser (#501)

- Replaced libxml references with Xerces.
- Removed dead code.
- Add xerces-c project back into dependencies.  This was pulled in 11/2017, but we're switching to using it in product code now
- Templatize function pointers using std::function
- Remove unused endPdx method
- Rename member variables to conform to our conventions
- Move getFactoryFunc into Utils where it belongs
- Added negative test case to verify we assert if missing required attribute
- GEODE-6914: Apply correct compiler settings to Xerces
- GEODE-6914: Disable schema validation for the time being
- Add alias for xerces-c to CMakeLists, update reference to it
- Mark Xerces headers as system headers
- Spell out 'function' in getFactoryFunction
- Corresponding formatting changes
- Add a CacheXmlParser test that loads a cache.xml with a bad schema and assert it doesn't throw, i.e. we're not validating schemas

Co-authored-by: Matthew Reddington <mreddington@pivotal.io>
Co-authored-by: Mike Martell <mmartell@pivotal.io>
Co-authored-by: Jacob Barrett <jbarrett@pivotal.io>