Add hif_transaction_get_db()
Add hif_db_ensure_origin_pkg()
Export hif_transaction_ensure_source()
Use a default cache age of 7 days
Only set up the package sack when required

This means that PackageKit doesn't load all the metadata when launched.
trivial: Use a better error message in hif_transaction_ensure_source()
Reset the transaction after it has been used