not adding a duplicate printer but still use it

[Antoine Martin]

Caused by #2095.
When we find that the printer already exists on the server, we don't add it again.
Then when we try to print to it, the server tries to find the client that added it and doesn't find it... and nothing gets printed.
Looks like we need to keep track of the printers we meant to add separately, so we can find the client to send the job to.

[Antoine Martin]

Not really a problem I think.
We match the uuid not the printer name, and the code just needed fixing for python3: r22366.