Limit the number of items we try to sync at once

Throttling the sync actually provides a better UX
because it gives the item list a chance to update
during the sync. Previously the item list was not updating
until the entire sync had completed.
Delete the local data store without requiring an app reload

When activating 'Clear Offline Cache', close the database
before deleting it, otherwise the existing DB is kept around
until the app is reloaded.