system: Add worker_id to process_name when workers is larger than 1

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Signed-off-by: Masahiro Nakagawa repeatedly@gmail.com

Which issue(s) this PR fixes:
No entry.

What this PR does / why we need it:
Currently, worker's process names are same when workers > 1. This patch improves such case like below:

% ps aux | grep fluentd
repeatedly        7923   0.0  0.1  2521484  21240 s010  S+    4:25AM   0:00.83 worker:fluentd3     
repeatedly        7922   0.0  0.1  2519436  17984 s010  S+    4:25AM   0:00.82 worker:fluentd2     
repeatedly        7921   0.0  0.1  2522520  18676 s010  S+    4:25AM   0:00.86 worker:fluentd1     
repeatedly        7920   0.0  0.1  2522520  23420 s010  S+    4:25AM   0:00.88 worker:fluentd0     
repeatedly        7892   0.0  0.1  2523544  10260 s010  S+    4:25AM   0:00.65 supervisor:fluentd 

Docs Changes:
Need multi worker article update.

Release Note:

See PR title.

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@okkez Could you also check this patch?

[okkez]

Sorry for the delay.
Looks nice!