Demote a warn when enabling CellStatistics

When the CellStatistics option is off, we don't store cell insertion
times. Doing so would also not be very smart, because there seem to
still be some performance issues with this type of statistics. Nothing
harmful happens when we don't have insertion times, so we don't need to
alarm the user.

changes/bug1921

@@ -0,0 +1,4 @@
+  o Minor bugfixes:
+    - Demote a warn to info that happens when the CellStatistics option
+      was just enabled. Fixes bug 1921; bugfix on 0.2.2.1-alpha. Reported
+      by Moritz Bartl.

src/or/relay.c

@@ -2297,7 +2297,9 @@ connection_or_flush_from_first_active_circuit(or_connection_t *conn, int max,
       flushed = (uint32_t)((now.tv_sec % SECONDS_IN_A_DAY) * 100L +
                  (uint32_t)now.tv_usec / (uint32_t)10000L);
       if (!it_queue || !it_queue->first) {
-        log_warn(LD_BUG, "Cannot determine insertion time of cell.");
+        log_info(LD_GENERAL, "Cannot determine insertion time of cell. "
+                             "Looks like the CellStatistics option was "
+                             "recently enabled.");
       } else {
         or_circuit_t *orcirc = TO_OR_CIRCUIT(circ);
         insertion_time_elem_t *elem = it_queue->first;