Add 'omero.import.active_close' system property

The primary blocker for this PR as a feature for 5.0.3
was that it broke compatibility with 5.0.2 servers and
earlier. Now the active closing functionality which is
needed for in-place import on Windows is *disabled* by
default, and needs to be activated via:

```
JAVA_OPTS=-Domero.import.active_close=true bin/omero...
```