fix(migrations) makes migrations more reentrant
fix(migrations) makes migrations more reentrant

Improves reentrancy of migrations, by making sure each
operation has its own exception. In practice this means
avoiding constructs such as:

```
DO $$
BEGIN
  ALTER TABLE IF EXISTS ONLY "routes" ADD "name" TEXT UNIQUE;
  ALTER TABLE IF EXISTS ONLY "routes" ADD "snis" TEXT UNIQUE;
EXCEPTION WHEN DUPLICATE_COLUMN THEN
   -- Do nothing, accept existing state
END;
$$;
```

in which "name" was declared in one release candidate
and "snis" in the next. By sharing the same exception, running
this migration on the latter rc would throw on the first `ALTER
TABLE` and never run the second one.

It also ensures that `run_on` is added to the Cassandra migration
during the `up` phase.

Kong/kong,Mashape/kong,Kong/kong,Kong/kong