Specify full number of images to load

If the full number is not specified, the state will be modified
before the loading is complete.
Sort the images before loading.
Do not display progress bar.
Correct more concurrent modifications
Make `networkup` thread-safe

The call to isNetworkup was first setting networkup
to false before setting the value, but this was a
race condition: if one thread checked the value before
the other thread had finished calling, then a false
negative was returned. This would likely explain many
of the network-down messages we have seen.
Mark for possible removal
Better handling of recreate/permitNull flags

There were cases where a null Connector could be returned
from OMEROGateway.getConnector. That should now be fixed.
Clearer messages are now logged/thrown.
Remove NetworkChecker from RenderingControlProxy

Since the proxy has access to a service, add isAlive to the
service interface and re-use the OMEROGateway.getConnector
logic from the proxy.
Add logging to NetworkChecker