Don't use rowids as foreign key into fulltext values table

[ncalexan]

Per http://www.sqlabs.com/blog/2010/12/sqlite-and-unique-rowid-something-you-really-need-to-know/ and others, VACUUM can re-assign rowids. At this time, we don't ever delete fulltext values, so this is a theoretical concern; but we should fix it, possibly by adding an INTEGER PRIMARY KEY AUTOINCREMENT to the fulltext values table.

[rnewman]

“The rowid of an FTS table behaves in the same way as the rowid column of an ordinary SQLite table, except that the values stored in the rowid column of an FTS table remain unchanged if the database is rebuilt using the VACUUM command. For FTS tables, "docid" is allowed as an alias along with the usual "rowid", "oid" and "oid" identifiers.”