Fix excessive stack usage
Fix excessive stack usage

Fixes warning C6262 (/analyze). rapidxml::xml_document stores the data
in a stack-allocated buffer which then can trigger warning above and
might lead to a stack overflow exception on Windows.

Solution: wrap xml_document with a unique_ptr moves everything to the
heap. Code is still not finished (race condition, space overhead).

otris/ews-cpp,otris/ews-cpp,otris/ews-cpp,otris/ews-cpp,m-Schlitzer/ews-cpp,bkircher/ews-cpp,bkircher/ews-cpp,m-Schlitzer/ews-cpp,bkircher/ews-cpp,m-Schlitzer/ews-cpp