Use expat SAX parser instead of boost::ptree (DOM)

Use expat SAX parser instead of boost::ptree (DOM)

Modified xml parsing methods to use expat parser, which is much
faster than boost::property_tree (initialization time decreased
by about 30%). Added missing includes after the removal of ptree.

Change-Id: I8973fe7ad7bbfe7928ef261fe07305f54e640b76