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
should there not be some exception handling here in case we do exceed the number of connection? (not sure how this is displayed to the user, or how things are retried...)