Summary:
This patch is similar to {D28178}. Although `uid` is populated, and we can
freely dispatch both contact and photo modify requests in parallel, the
following race condition still sometimes happens:

 1) Contact modify request dispatched
 2) Contact photo modify request dispatched
 3) Photo reply arrives

At this point, `d->lastContact` is not yet populated, and we call
`processNextContact()`, which repeats steps 1 and 2 for the very same contact.
Which could happen again and again, if photo reply arrives first again.

Instead, I suggest to fire photo modify request only after we receive reply
for contact modify request, ensuring everything happens in the following order:

 1) Contact modify request dispatched
 2) Contact modify reply received
 3) Contact photo modify request dispatched
 4) Contact photo reply received

Test Plan:
 1) Modify a contact via KAddressBook
 2) Check logs
 3) (without patch) Bunch of modify requests somtimes gets fired
 4) (with patch) Only one modify request gets fired

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D28181