DS-2267 SOLR usage statistics to export to common format

https://jira.duraspace.org/browse/DS-2267

It was mentioned that we need a way to export SOLR's data, so we could also import it. Well, we've already been using some statistics intermediate format to go from the DSpace.log to SOLR, i.e. stats-log-converter, so this PR output's SOLR's statistics in that same format. You could then reimport into SOLR, or import into Elastic Search

/dspace/bin/dspace stats-util --export
There are 70 usage events in SOLR for download/view.
Export hits [0 - 9999] to /dspace/temp/usagestats_0.csv