Resolve all warnings in CI

[tagomoris]

Currently, Fluentd code reports many warnings if it is enabled.
It shows potential errors and troubles in code. So warnings should be resolved basically.

This ticket is blocker for v0.14.0 release.

1. revert Rakefile: Suppress warnings from ruby, enabled by rake 11 #846
2. remove all warnings from fluentd's code
3. remove warnings from dependencies as much as possible

[okkez]

See also https://bugs.ruby-lang.org/issues/12026

[okkez]

NOTE:

• lib/fluent/plugin/socket_util.rb:61: warning: instance variable @timeout not initialized
• https://github.com/fluent/fluentd/blob/master/lib/fluent/plugin/socket_util.rb#L61
• https://github.com/fluent/fluentd/blob/master/lib/fluent/plugin/in_stream.rb#L120

I think we cannot set @timeout from outside of the class.
(I think we can replace @timeout with 0 or initialize @timeout with 0.)

---

lib/fluent/test/input_test.rb:112: warning: method redefined; discarding old emit_stream
lib/fluent/test/input_test.rb:112: warning: previous definition of emit_stream was here
lib/fluent/test/input_test.rb:115: warning: method redefined; discarding old emit_stream
lib/fluent/test/input_test.rb:115: warning: previous definition of emit_stream was here
lib/fluent/test/input_test.rb:112: warning: method redefined; discarding old emit_stream

• https://github.com/fluent/fluentd/blob/master/lib/fluent/test/input_test.rb#L110
  • Invoke Fluent::Test::InputTestDriver#run multiple times in a process
  • Fluent::Engine and instance.router is singleton object

[tagomoris]

#966 will solve this problem.

[tagomoris]

After #966, there are no warning shown in running tests. There are still some warnings in loading scripts, but I think it's not serious blocker for the project.
(For other committers: If it's wrong, please reopen this.)