Fixed unsafe use of localtime

Since localtime is not thread-safe it should not be used in multithreaded
contexts. For this reason all calls to localtime were changed to localtime_r
in code where concurrency issues were possible.

Internal tests were left unchanged because they aren't multithreaded.

log_manager/test/testlog.c

@@ -161,7 +161,7 @@ int main(int argc, char* argv[])
     ss_dassert(succp);
 
     t = time(NULL);
-    tm = *(localtime(&t));
+    localtime_r(&t, &tm);
     err = MXS_ERROR("%04d %02d/%02d %02d.%02d.%02d",
                     tm.tm_year+1900,
                     tm.tm_mon+1,