Close ImportProcessPrx instances (Fix #12233)

Once upload is completed, the import process instance should
be closed. The `HandlePrx` which is returned will continue
functioning.

For testing:
 * `touch a.fake`
 * `bin/omero import a.fake`
 * `bin/omero sessions group SOMEOTHERGROUP`  # Previously this would fail

/cc @jburel - better suggestion on logging inside of OMEROGateway?