MINOR: log: add the %Td log-format specifier

As suggested by Pavlos, it's too bad that we didn't have a %Td log
format tag given that there are a few mentions of Td corresponding
to the data transmission time already in the doc, so this is now done.
Just like the other specifiers, we report -1 if the connection failed
before reaching the data transmission state.
diff --git a/include/types/log.h b/include/types/log.h
index e974086..25d872e 100644
--- a/include/types/log.h
+++ b/include/types/log.h
@@ -81,6 +81,7 @@
 	LOG_FMT_TW,
 	LOG_FMT_TC,
 	LOG_FMT_TR,
+	LOG_FMT_TD,
 	LOG_FMT_TT,
 	LOG_FMT_STATUS,
 	LOG_FMT_CCLIENT,