Refactor SlaveInterface::calcSpeed

Summary:
Use QVector instead of a static array.
Use QElapsedTimer instead of QDateTime for elapsed time.

Test Plan:
Copied several large files(5-20Gb). The difference between current
and previous speed calculation was within 0-300Kb.

Reviewers: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D17528