Only create checksums for non-existing checksums

This commit fixes an issue that results in error message spam on
the PostgreSQL server. We were previously attempting an insert on
the checksums table for all checksums that belong to a cookbook, even
if they already existed in the database. Because we were rescuing the
conflict, we weren't actually seeing errors, just log spam from
PostgreSQL. This commit ensures that we don't send inserts for
checksums that we already know exist.

/cc @opscode/server-team @seth