Fluentd forwarder status page is displayed with a huge delay when aggregator node is responding slowly

OS: centos 7
Fluentd version: td-agent-3.2.0-0.el7.x86_64

When aggregator node is failing or responding very slowly while under heavy load, it might take up to 1-2 minutes to get a status page /api/plugins.json on a forwarder node.

Steps to reproduce

Forwarder config

```

@type monitor_agent
bind 127.0.0.1
port 24220

@type forward
bind 127.0.0.1
port 24224

@type forward

heartbeat_type tcp
send_timeout 60s
recover_wait 10s
heartbeat_interval 1s
# increased this while testing
phi_threshold 160000
hard_timeout 120s

name logs1
host 172.31.3.5
port 8889
weight 60

flush_interval 10s

buffer_type file
buffer_path /var/log/fluentd/buffer/forward
buffer_chunk_limit 4m
buffer_queue_limit 4096
num_threads 2
expire_dns_cache 600

```

I make some service send logs to the forwarder.

Then on aggregator node I execute

`# iptables -A INPUT -m statistic --mode random --probability 0.8 --source forwarder.node.ip.address -j DROP`

On the forwarder node I execute the following curl request in a loop

`# while true; do timeout 2 curl -s http://localhost:24220/api/plugins.json > /dev/null && echo ok || echo failure; sleep 1; done
`
In some time it starts showing "failure".

When I flush iptables rules on the aggregator node with

`iptables -F`

it gets back to normal.

It happens not all the time, but in a rather big percentage of cases it happens.

td-agent 2.5 is not affected.

Also I noticed that docker services that send logs to the forwarder stop responding sometimes as well. But was not able to reproduce it yet in my test environment.

Thanks.

Regards,
Sergey