Add utility function to check if an executor is direct or not

[krka]

Don't move futures to different executor if it's a direct executor

[coveralls]

Coverage remained the same at ?% when pulling c569754a687cb72648aa84d85d3c618b4eb0a7d7 on krka:krka/directexecutor into b7395b3c96b5d649ec6f5096b968400ac628e379 on spotify:master.