On-device database schema enhancements for sync

[linuxwolf]

In service to sync, the on-device IndexedDB requires additional collections/tables:

• pending to track changes to be applied (locally and remotely)
• markers to track the last sync point for each collection (items and keystores)

Further modifications to the existing keystores table are necessary:

• remove uuid from the indexes
• add id to the record (indexed), set to HASH(group)
• add last_modified to record (but not indexed) to track the server-provided timestamp

Additional changes to the items table are necessary:

• add last_modified to record (but not indexed) to track the server-provided timestamp

Related

• Cloud storage of saved credentials so that they can be accessed from any supported platforms (a single database of credentials for each user) #50