Completed FulltextUpdaterWork should clean its data

Completed job are kept until periodic cleaning (see clearCompletedAfterSeconds in org.nuxeo.ecm.core.work.service queues EP).

When doing mass import the number of completed jobs can be high and worker should minimize their memory footprint on completion.

The FulltextUpdaterWork keeps many FulltextUpdaterInfo in its context.

Because of this mass import using nuxeo-importer is GC bound.