library item versioning

[msfrank]

in order for the daap server to perform updates (let a client know that the library has changed in some way), we need a way to know what items were added when. we can do this by adding a new database table, Version, which has two columns, id and date. id will be an autoincrementing unsigned int, and date is the date of the row creation.

we will use Version like a transaction wrapper: each time we want to make db modifications, we create a new row in Version, and each added/modified/deleted(?) library item will be tagged with the new version. then, when a particular db version is requested by a daap client, we can walk the database returning each item which has a version equal to or less than the supplied version.