Parse XML docs using a reader, don't load the whole DOM at memory.

Parse XML docs using a reader, don't load the whole DOM at memory.

+Use 'libxml/xmlreader.h' functions to validate and parse XML documents, this decreases the use of memory that was being allocated using other functions.
+Update the logic at 'rrdp_parser.c' to parse a document element by element, using an 'xmlTextReader'.
+Update unit test to use the XML text reader.