cassandra: Deduplicate table names
This removes all table names from cassandra/cql.py, and gets them from cassandra/schema.py instead.
When possible, this uses existing constants (BaseRow.TABLE), otherwise it uses a function to compute these names.
This is needed to support schema migrations, as updating a table's primary key requires creating a new table with a different name.
Prerequisite for #2602 (closed).
Migrated from D5574 (view on Phabricator)