Cache std::random_device

Summary: Creating or opening a std::random_device incurs an overhead one can see in profiling (about 7% on OS X; on Linux and OS X, this causes a call to open()).

Reviewers: Krita, dkazakov

Reviewed By: Krita, dkazakov

Subscribers: dkazakov

Differential Revision: https://phabricator.kde.org/D7895