10ms time precision via EXSLT date-time function

z3t reported at HackerOne that the EXSLT date-tme function is subverting our patches that set the timing granularity to 100ms. We get at least a 10ms precision that way. PoC is on https://people.torproject.org/~gk/tests/tor_xml_time.html. And see dom/xslt/xslt/txEXSLTFunctions.cpp for the date:date-time implementation.