Sporadic app lock up when syncing

[smesdaghi]

Arbiter locks up when syncing some layers sometimes on a good connection network.

[smesdaghi]

Created a project with od3 layers and noticed app locks up. tried various layers with media sync enabled/disabled. Turned out to be if the first feature on the layer happens to have a null datetime attribute, the DownloadedFeaturesHandler.checkTimeDifference method was not catching a a couple of cases resulting in sync not proceeding. We need a better solution that the current implementation for what looks like time difference between server and device computation that doesn't rely on a wfs check for first feature on layer. pushed the patch for now.