[contacts/ContactModifyJob] Avoid race condition

Summary:
This patch is similar to D28178: [contacts] Fix ContactCreateJob with a photo. 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