Add method to update the latest index when a CPAN module is uploaded

[preaction]

Every time a record is written to the uploads table, we need to make sure that the ixlatest table is up-to-date. This is the most common thing to do when inserting new records, so we should make it easy.

Make a method in CPAN::Testers::Schema::ResultSet::Upload called insert_index that inserts the new upload into the database, and then also updates the correct row in CPAN::Testers::Schema::Result::LatestIndex (but only if the upload is newer, see the existing backend code for when ixlatest is updated).

The method should then return the new CPAN::Testers::Schema::Result::Upload object.

This method is required for the CPAN Testers backend (cpan-testers/cpantesters-backend#13), and also makes syncing data from the database easier (#20)