fix #11360: get the servant holder from the session factory into the raw file store

fixes http://trac.openmicroscopy.org.uk/ome/ticket/11360

Check that the appropriate OMERO integration test Jenkins job passes or try running tests locally. A quick start would be,
```
./build.py -f components/tools/OmeroJava/build.xml test -DTEST=integration/RawFileStoreTest
```
but run the other non-broken ones too if Jenkins didn't do it for you (timeouts, whatever).