[contacts] Fix ContactCreateJob with a photo

Summary:
When we run `ContactCreateJob`, the UID of a contact is not yet known.
We can only add a photo only after we receive a reply from main create job.

Also, since we also don't want the following race condition to trigger:
 1) Contact1 create request
 2) Contact1 reply, Photo1 change request, Contact2 create request
 3) Contact2 reply, Photo2 change request
 4) Photo1 reply (here both `currentContact` and `lastContact` is Contact2)
 5) Photo2 reply
we only go for next contact after the previous has been fully procesed:
 1) Contact1 create request
 2) Contact1 reply, Photo1 change request
 3) Photo1 reply, Contact2 create request
 4) Contact2 reply, Photo2 change request
 5) Photo2 reply

Test Plan:
Without patch:
 1) Create a contact with photo inside KAddressBook
 2) Check Web UI: no photo is set, sometimes bunch of contacts get created (?)
With patch:
 2) Observe a single contact with a proper photo being set

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D28178