Fix sync image race/hang for syncimage_status#299

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vehre wants to merge 3 commits intomasterfrom

vehre/sync_image_fix

Commits on Dec 29, 2016

Null array-descriptor in mpi-token causing segfault on finalize.
vehre
committed

Commits on Dec 30, 2016

Fix sync image race condition.
vehre
committed

Commits on Jan 2, 2017

Ensure images status is written during an exclusive lock is held only.
vehre
committed