11074 export close

Correct the underlying issue with `preClose` not being called on `ExporterI` as well as print out info regarding the size of the temp directory from `bin/omero admin diagnostics`. To test, export a large file and check that its not present under the server temp directory. E.g.:
```
jamoore@blue:/opt/ome2/dist$ ls -ltra ~/omero/tmp/omero_jamoore/
total 124
...
dwxrwxr-x  2 jamoore jamoore 4096 Jun  9 21:30 11771@blue  <-- last entry
```

Also run `bin/omero admin diagnostics` both with and without `OMERO_TEMPDIR` set to see if the proper size is shown.