ItemSync: speed up by not using takeFirst().

Summary:
perf+hotspot shows that this is a horrible algorithm:
http://www.davidfaure.fr/kde/perf_hotspot_itemsync.png

I ported the code to the std::move (the algorithm)

Test Plan:
Added a benchmark to itemsynctest.

Before:

RESULT : ItemsyncTest::testFullSyncManyItems():
    368,935,368.19 CPU cycles per iteration (total: 36,893,536,820, iterations: 100)

After:

 RESULT : ItemsyncTest::testFullSyncManyItems():
     349,899,659.49 CPU cycles per iteration (total: 34,989,965,949, iterations: 100)

=> 5% faster (for the whole of ItemSync, which runs much more than this piece of code)

Went down to 346 million after https://commits.kde.org/kcoreaddons/42e1d6e8e7bf7f2738fbe15778b23a84c104e8a8

(Qt + Akonadi built in release-with-debug; KF5 in debug, but not much KF5 code being run here)

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: mwolff, KDE PIM

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D8465