BAZYNGO-705: let ResourceProvider return QueryResults. This allows
checking total row count for progress reporting without running the
query twice.
BAZYNGO-702: error checking - rollback on RuntimeException, pass
exception to the caller
BAZYNGO-702: We shouldn't hold onto an IndexWriter, because a rollback()
closes it and makes it unusable. Use IndexReaders / IndexWriters created
on demand in try-with-resource clauses instead.
BAZYNGO-702: moved reindex task implementation to GenericIndex