Add keep alive to tcp plugin

[dimitarKiryakov]

Which issue(s) this PR fixes:
Fixes #3960

What this PR does / why we need it:
TCP keepalive should be supported by in_tcp to avoid dead connection, as same as in_forward

Docs Changes:
send_keepalive_packet should be added to https://docs.fluentd.org/input/tcp
Release Note:
Same with the title

[dimitarKiryakov]

@fujimotos could you please approve running the workflows?

[fujimotos]

@dimitarKiryakov Thank you for contribution!

TCP keepalive should be supported by in_tcp to avoid dead connection, as same as in_forward

I eyeballed your patch. It seems good, but I want to do a bit of more
testing on it tomorrow. Wait for me.

Could you please approve running the workflows?

BTW, I approved the workflow run. The tests have just started running.

[dimitarKiryakov]

@fujimotos Thanks a lot for your help on this. This feature is very crucial for us as it will mitigate the issue of dead connections. I think that the currently failing tests are not related to my change. Did you manage to test it?

Thank you a lot!

[fujimotos]

Approved. I can confirm this patch works.

[fujimotos]

Here is how I tested this patch. First, I created a simple TCP server
using the configuration attached below:

$ fluentd -c tcp.conf

I initiated a TCP connection to the server in another terminal, and kept
it open for a while:

$ telnet localhost 9000

Then I monitored the network stream using tcpdump:

$ tcpdump
...
14:20:21.470600 IP 127.0.0.1.40446 > 127.0.0.1.9000: Flags [.], ack 1, win 512, options [nop,nop,TS val 1282790546 ecr 1282790546], length 0
14:21:22.182178 IP 127.0.0.1.9000 > 127.0.0.1.40446: Flags [.], ack 1, win 512, options [nop,nop,TS val 1282851258 ecr 1282790546], length 0
14:21:22.182196 IP 127.0.0.1.40446 > 127.0.0.1.9000: Flags [.], ack 1, win 512, options [nop,nop,TS val 1282851258 ecr 1282790546], length 0
14:22:23.622148 IP 127.0.0.1.9000 > 127.0.0.1.40446: Flags [.], ack 1, win 512, options [nop,nop,TS val 1282912698 ecr 1282851258], length 0

As shown above, keepalive packets are being transmitted after some
period of inactivity. So I believe the feature is working as expected.

tcp.conf

<source>
  @type tcp
  port 9000
  format none
  tag "test.log"
  send_keepalive_packet true
</source>
<match test.**>
  @type stdout
</match>

[fujimotos]

Merged via 66c4a8b.

Note: I described more details in the commit message and also
added a little tweak to the test case.

[fujimotos]

Documentation added in fluent/fluentd-docs-gitbook#438