Revamped DomDocumentCache with regard to thread safety

* DomDocumentCache::item class now is _finalized_ by default and doesn't perform document instantiation, just lifetime management
** xercesc::DOMDocument is instantiated inside the external function implementations and committed to the document cache for conversion to a xalan document
* added mutex with scoped lock to prevent concurrent write access to the std::unordered_map contained withing DomDocumentCache
* functionality of the _get_ member method was split into _get_ and _create_
** added typedef for std::pair specialization type "optional_item" that functions as the return value of _create_ and _get_
* "locator->getSystemId()" was leaking memory as xerces doesn't manage the lifetime of the returned heap-allocated char array
** analog to XMLCh* strings
** transformed XercesStringGuard into template class to be instantiated on either XMLCh or char

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