cubicweb #1908676 gracefully handle synchronization interrupts [resolved]

If the app is synchronizing an external datafeed source and is shut down, the "current synchronization" attribute of the source will stay to "yes" and next time the instance is restarted, the synch will never ever happen again because the app will think the synch is in progress.

done in3.13.8
load left0.000
closed by#a93e2ed5877a [datafeed] add max-lifetime for concurrent synchronization lock (closes #1908676)