commons: fix TimeUtils nanos to duration string conversion

Ached-by: Karsten Schwank
Target: master
Require-book: no
Require-notes: no
(cherry picked from commit ae63e2ed2657066599a09280737a07f56e7e1760)
Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Merge pull request #856 from kofemann/fix-timeutils-2.10

commons: fix TimeUtils nanos to duration string conversion