Allow importer to work in non-bulk mode

Today the importer is automatically setting the event service in bulk mode during import.

This may be undesirable (for instance when we still want regular Work instances to be created for async events during import), so a flag to disable bulk mode for the importer would be nice to have.

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With the following contribution one can now configure the importer to work in non-bulk mode, which is a bit slower but allows regular Work instances to be created and directed to specific queues:

  <extension target="org.nuxeo.ecm.platform.importer.service.DefaultImporterComponent" point="importerConfiguration">
    <importerConfig>
      <bulkMode>false</bulkMode>
    </importerConfig>
  </extension>