Close ImportProcessPrx instances (Fix #12233) (rebased onto develop)



This is the same as gh-2395 but rebased onto develop.

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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?