wrong shutdown event object handling in plugins

[colinsurprenant]

while working on performance issues I discovered what is probably the source of the pipeline shutdown stalling regression in 1.5 we've been seeing.

The plugins are checking for LogStash::SHUTDOWN that is initialized as the :shutdown symbol while the pipeline pushes a LogStash::ShutdownEvent.new so the plugins never see the shutdown event.

The fix is currently in this commit c07368c which is part of the fix/perf_regression shared branch.

Once we merge that branch we'll have to release a new logstash-core version.

[colinsurprenant]

fix included in the #2869 WIP PR

[colinsurprenant]

@jsvd please take a look, as it explains the pipeline shutdown problems in 1.5

[colinsurprenant]

this was closed by #2869