Signal external syncs to stop during backup/restore
This change is intended to prevent syncs occuring during backup/restore.
When syncing is delegated to external processes (e.g. messageserver5 handles the AlwaysOn case for IMAP), buteo controls this by sending out syncedExternallyStatus dbus signals. The external processes should react to this signal appropriately.
This change makes it so that at the start of a backup or restore operation, signals are sent out to external processes indicating that external syncs should stop.
During the backup/restore, changes to the externalSync status are stored in the account info, but not acted on.
At the end of the backup/restore, syncedExternallyStatus dbus signals are sent out for all profiles that should be syncing externally requesting the external syncs to be resumed.