Rename object stores from 'key-store' and 'item-store' to 'keys' and 'items'

The '-store' part of the name was redundant.
This also takes care of wiping old data when upgrading to the
new DB format which multiple revisions of an item's content.
Add comments with schema documentation for local_store.Store
Add method to get last-synced timestamps for all items

For efficient testing of whether an item changed
locally or in the vault since the last sync, add a method
to SyncableStore to retrieve a (item ID -> last synced at timestamp)
map for all items in the store and implement for local_store.Store
and temp_store.Store
Fix test for whether an item changed in store or vault since last sync

Compare the updatedAt timestamps for the store and vault items
to the last-synced timestamp rather than comparing the
store/vault updatedAt timestamps directly - since these could
have been changed at the same time but with different changes.
Implement basic overview data merging for local and cloud vault changes
Suppress sync log messages

We'll need a proper categorized logging system in future.