Fix race when accessing std::locale::classic()

See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78552

src/utiltime.cpp

@@ -74,8 +74,9 @@ void MilliSleep(int64_t n)
 
 std::string DateTimeStrFormat(const char* pszFormat, int64_t nTime)
 {
+    static std::locale classic(std::locale::classic());
     // std::locale takes ownership of the pointer
-    std::locale loc(std::locale::classic(), new boost::posix_time::time_facet(pszFormat));
+    std::locale loc(classic, new boost::posix_time::time_facet(pszFormat));
     std::stringstream ss;
     ss.imbue(loc);
     ss << boost::posix_time::from_time_t(nTime);