Restart the sync when we detect we need to redo a sync

Fixes #1968
Relates #2038

(cherry picked from commit a84b7dc27e43c9e9ef96fa8d25a561ee058f89c7)

Conflicts:
	src/mirall/owncloudpropagator.h
	src/mirall/propagator_qnam.cpp
	src/mirall/syncengine.h