set concurrency to single #21
                
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Part of the meta issue elastic/logstash#5663
this PR updates the plugin output delegation strategy to conform with elastic/logstash#5752
Using
concurrency :singlemakes a single instance of the plugin to be shared accross all pipeline workers. However only 1 will be able to use the plugin at a time.See #20 for a longer running rework to make the plugin be threadsafe, in order to set
concurrency :shared.