Fix disabling unavoidable perf logging for importers

Customer witnesses a file perflog_[0-9]*.csv générated at every instantiation of an importer in nxserver, in relation with the timer <cronExpression> parametrizable in NUXEO. ( see https://doc.nuxeo.com/nxdoc/scheduling-periodic-events/#cron-expression )

There is currently no way to avoid the creation of this file in https://github.com/nuxeo/nuxeo-platform-importer/blob/master/nuxeo-importer-core/src/main/java/org/nuxeo/ecm/platform/importer/base/GenericMultiThreadedImporter.java

Customer requests that this file creation (several times per minute) can be disabled through parameterisation.