Reload new config without restarting process

[ganmacs]

Which issue(s) this PR fixes:
Fixes #2624

What this PR does / why we need it:

This change make fluentd be able to reload new config without restarting process with SIGUSR2 signal.

it's very lighter and safer than existing reloading feature. but it has 2 limitations.

1. A change to system_config is ignored because it needs to restart(kill/spawn) process.
2. All plugins must not use class variable when restarting.

Docs Changes:

Add USR2 description.

Release Note:

same as title

[cosmo0920]

I feel StaticConfigAnalysis gives us special powers to report misconfiguration checking phase. 💪
I've added two comments for small concerns.

[ganmacs]

@repeatedly ?

[repeatedly]

I noticed we need to update HTTP RPC for this feature.
Should we change this to use USR2 or add new path?

fluentd/lib/fluent/supervisor.rb

Line 106 in c809788

@rpc_server.mount_proc('/api/config.reload') { |req, res|

[cosmo0920]

Should we change this to use USR2 or add new path?

I believe that we should add a new path for reloading config.
Windows does not have USR2 signal:

• https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/signal?view=vs-2019

[ganmacs]

I noticed we need to update HTTP RPC for this feature.
Should we change this to use USR2 or add new path?

Good catch! I forgot about it. I added new endpoint /api/config.gracefulReload(d8b8bc6) for compatibility.

Windows does not have USR2 signal:

I think this feature can't support windows like USR1 because a supervisor needs to send USR2 signal to workers to do this. https://github.com/fluent/fluentd/pull/2716/files#diff-dbe0e1ec4079138e48ca6a4d7c7248f9R215
We need to introduce another way to communicate between supervisor and workers without using a signal to be able to use this feature in windows.

[repeatedly]

I will release v1.9.0.rc1 soon.