pt-heartbeat doesn't REPLACE by default

I believe this is legacy left over from when I cared about the tool working by default on PostgreSQL. But it will be safer for replication if we use --replace by default. This will help avoid problems with missing rows on replicas (for whatever reason). It won't eliminate every possible problem on multi-tier replication topologies with row-based replication enabled (because the SBR statements will then get converted to RBR and logged as an update to an existing row), but it will help in some cases.