Sentinel: fix delay in detecting ODOWN.

See issue #2819 for details. The gist is that when we want to send INFO
because we are over the time, we used to send only INFO commands, no
longer sending PING commands. However if a master fails exactly when we
are about to send an INFO command, the PING times will result zero
because the PONG reply was already received, and we'll fail to send more
PINGs, since we try only to send INFO commands: the failure detector
will delay until the connection is closed and re-opened for "long
timeout".

This commit changes the logic so that we can send the three kind of
messages regardless of the fact we sent another one already in the same
code path. It could happen that we go over the message limit for the
link by a few messages, but this is not significant. However now we'll
not introduce delays in sending commands just because there was
something else to send at the same time.

src/sentinel.c

@@ -2599,20 +2599,24 @@ void sentinelSendPeriodicCommands(sentinelRedisInstance *ri) {
     ping_period = ri->down_after_period;
     if (ping_period > SENTINEL_PING_PERIOD) ping_period = SENTINEL_PING_PERIOD;
 
+    /* Send INFO to masters and slaves, not sentinels. */
     if ((ri->flags & SRI_SENTINEL) == 0 &&
         (ri->info_refresh == 0 ||
         (now - ri->info_refresh) > info_period))
     {
-        /* Send INFO to masters and slaves, not sentinels. */
         retval = redisAsyncCommand(ri->link->cc,
             sentinelInfoReplyCallback, ri, "INFO");
         if (retval == C_OK) ri->link->pending_commands++;
-    } else if ((now - ri->link->last_pong_time) > ping_period &&
+    }
+
+    /* Send PING to all the three kinds of instances. */
+    if ((now - ri->link->last_pong_time) > ping_period &&
                (now - ri->link->last_ping_time) > ping_period/2) {
-        /* Send PING to all the three kinds of instances. */
         sentinelSendPing(ri);
-    } else if ((now - ri->last_pub_time) > SENTINEL_PUBLISH_PERIOD) {
-        /* PUBLISH hello messages to all the three kinds of instances. */
+    }
+
+    /* PUBLISH hello messages to all the three kinds of instances. */
+    if ((now - ri->last_pub_time) > SENTINEL_PUBLISH_PERIOD) {
         sentinelSendHello(ri);
     }
 }