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refactor(plugin): create one plugin instance per invocation #613

Job	Run time
discover conformance suites	10s
conformance (general)	1m 53s
conformance (parallel)	1m 51s
conformance (wait)	1m 36s
conformance (wait_for_condition)	3m 22s
conformance (step)	2m 29s
conformance (child)	2m 13s
conformance (callback)	2m 48s
conformance (plugin)	5m 33s
conformance (map)	1m 48s
conformance (wait_for_callback)	2m 5s
conformance (invoke)	3m 56s
	29m 44s