long getDirDeleteCount() {
            return dirDeleteCounter.sum();
        }
Deletes a batch of simpleMetas from the file system and the database.

@param deleteThresholdEpoch The threshold for statusTime, i.e. records <= deleteThresholdEpoch
@param progress             Count of failures that spans multiple batches
@return Those simpleMeta items that could not be deleted for some reason.
stroom-data/stroom-data-store-impl-fs/src/main/java/stroom/data/store/impl/fs/PhysicalDeleteExecutor.java