Fix chunked parsing of literals in literal body

Switch to use an appending stringbuffer to avoid
truncated/overwritten literals when the element content was delivered
in multiple literal chunks - especially a problem near unicode.

(rasqal_sparql_xml_sax2_start_element_handler): Initialise sb.

(rasqal_sparql_xml_sax2_characters_handler): Add content to sb

(rasqal_sparql_xml_sax2_end_element_handler): Form single value /
value_len vars from sb content.  Use it for literal values and
checking boolean.  Reset sb at end

(rasqal_sparql_xml_free_context): Free any sb.
less debugging, protected by TRACE_XML or higher RASQAL_DEBUG
Less literal comparison debugging, now RASQAL_DEBUG > 1
(rasqal_row_compare): Less debugging, now RASQAL_DEBUG > 1