Integrate Datastore sync implementation to allow users to retrieve and store their entries in the cloud

[linuxwolf]

In service to sync, integrate the core sync process from the datastore into the extension.

Todo

• [ ] make appropriate calls on DataStore instance to trigger a sync.
  • on sync message (manual trigger hook)
  • on upgrade_account message
  • on add_item / update_item / remove_item messages
  • on a timer
• [ ] report any sync errors to logs.
• [ ] record telemetry when the sync operation completes.

Related

• mozilla-lockbox/lockbox-datastore#50
• blocked by #487