Bugfix: Import As freezes

[dominikl]

Fixes Ticket 12929
Problem: After an object has been saved the connector was shutdown. This could lead to an abortion of a parallel running import job. This PR removes the connector shutdown, because it's actually not necessary at that point anyway.

Test:
Follow the workflow which previously triggered this bug, which is:

• Log in as a group owner or administrator
• Start (not just add to the queue) a long running import job for another user
• Now start another import for the user you're logged in with, but import into a particular dataset which you have to create first on the import dialog (this caused the crash)
• Make sure the imports are passing like expected.

Note: The actual bugfix is just commit 979098b . The first commits add PropertyChangeSupport to the Gateway, i. e. one can add a PropertyChangeListener to the Gateway for debugging purposes, which makes it much easier to monitor what's going on in the Gateway. Any objections on this?

/cc @jburel

[joshmoore]

[nomemoryjava] /home/travis/build/openmicroscopy/openmicroscopy/components/blitz/src/omero/gateway/Gateway.java:70: error: cannot find symbol
[nomemoryjava] import omero.gateway.util.GatewayMonitor;
[nomemoryjava]                          ^
[nomemoryjava]   symbol:   class GatewayMonitor
[nomemoryjava]   location: package omero.gateway.util

[jburel]

making debugging easier makes sense

[mtbc]

but it's just a boolean

[mtbc]

Functionally works fine, just a comment nitpick.

[mtbc]

Good to merge.

[jburel]

Merging so it does not conflict with login work done in gateway.