Database: Add CommitMarker table

This table stores change information such as the sortkey for the most
 recent changes that are fetched. With this data, we can limit the
 number of changes that are returned in the server response
 depending on when the last sync took place.
It makes sense to store this information in a seperate table as,
 although provided by the server, the sortkey is only used for the
 most recent changes stored in the database.

Change-Id: I983187f4983abea15aea9388dc70e2d1554750b8