This avoids a potential problem where the app crashes after adding the contact but before adding the transport keys, leaving the contact unusable.

Same exponential backoff approach as Bluetooth.

The polling interval increases exponentially each time polling is unsuccessful, up to a maximum of 60 minutes. The interval is reset to 2 minutes whenever a connection is made and whenever Bluetooth is re-enabled.

Don't keep message status rows for groups that are invisible to the contact - this avoids the need to join the groupVisibilities table when selecting messages to offer or send. Add or remove status rows when group visibility changes.