Enable suspend/resume for CSV Importer Work

A CSV import file may contain large numbers of records that may take quite some time to import. Since the back-end CSV Importer Work is asynchronous, it should be careful to gracefully tolerate a system shutdown or a disaster-recovery fail-over while the Work is in-progress. The CSV Importer Work already leverages the Transient Store to store per-record import results as well as to store the import blob. It should leverage this Transient Store state to resume the import after a shutdown or fail-over.

See the following excerpt from Nuxeo WorkManager document https://doc.nuxeo.com/nxdoc/work-and-workmanager/:

For long-running Work, work() should periodically check for isSuspending() and if true save the work state in non-transient fields of the Work instance and call suspended() and then return. This is how a server shutdown can interrupt a long-running Work, persist it, and relaunch it when the server restarts.