Use PRIMARY KEY for uid column

The database table creation statements set the uid column as an index. Since there should only be one row for a uid (user) the table should set the uid column as the PRIMARY KEY which will 1) enforce a unique value and 2) automatically make the column indexed.

Attached patch modifies the hook_install, adds some of the module variables to the hook_uninstall and adds a hook_update_1 function to drop the indexes and add the primary key.