parallelize InformerManager changeNamespaces #7292

Job	Run time
sample_operators_tests (sample-operators/webpage)	3m 28s
sample_operators_tests (sample-operators/mysql-schema)	3m 55s
sample_operators_tests (sample-operators/tomcat-operator)	4m 9s
sample_operators_tests (sample-operators/operations)	8m 43s
sample_operators_tests (sample-operators/leader-election)	4m 37s
	24m 52s

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