switch saxparser from expat to libxml2

Using SAX2 interface provides element/attribute names split into prefix/name,
and provides namespaces/urls, so FastSaxParserImpl::callbackStartElement()
does not have to figure out these on its own (and additionally libxml2
spends a noticeable portion in xmlStrdup() when not in SAX2 mode).
The switch saves about 5-10% time.

Change-Id: Idd424d7cc9b30c248179a5bad8ec79dbfc62e765