Add allow_deletes table schema option, which defaults to True

We see the need for this table schema option frequently in production systems, at both my current and previous job, to prevent disastrous zombie data cases where gc_grace_seconds is set to 0 for the life of the table. An example would be a write-only table with a default TTL and tombstones that won't clear fast enough.

Whenever I set gc_grace_seconds to 0, I typically update the comments to let any future users know that you shouldn't send deletes to that table, but I always fear that application developers will rarely read the production schema comments. When allow_deletes is set to False for a table, Cassandra would ideally throw an exception at ingestion time for delete mutations.

This would ensure that my previous assumption of a write-only table holds true as well as alert any future developers of that table's mutation restrictions.