delivery_carrier_label_dispatch - Improve efficiency of label generation by threading it.

For instance for postlogistics, using serialized calls to the SOAP web service
is a really slow as it would wait for a response before sending the next
request.

Also improve exception feed back. And thus exception are non blocking for all
label generation. You will only have to relaunch generation for failed ones.

Pre-requisite : 

- [x] https://github.com/OCA/carrier-delivery/pull/28

I moved this PR from Launchpad [0] on behalf of @yvaucher, he's the author.

[0] https://code.launchpad.net/~camptocamp/carriers-deliveries/7.0-threaded-dispatch-label-generation/+merge/215184
for the record you're filtering out empty labels here, and there is also a check for empty labels in `assemble_pdf` so you could probably dispense from the filtering here. (AFAIK "" <=base64=> "")