[RETENTION] Prevent multiple strategies operating on the same table

### About
The idea behind the composite primary key `PRIMARY KEY ("strategy", "table_schema", "table_name")` was to prevent duplicate strategies on the same table. Too bad we don't have `UNIQUE` constraints in CrateDB.

### Regression?
> Is there elsewhere in the code a check to prevent duplicates (i.e., for the same table, one entry with `DELETE` and 3 days retention, and another with `DELETE` and 5 days retention on the same table)?

_Originally posted by @hammerhead in https://github.com/crate-workbench/cratedb-retention/pull/20#discussion_r1250850198_