cassandra: Add annotations to make mypy actually type-check calls to CqlRunner.
All methods of CqlRunner were decorated, which prevented mypy from doing anything useful.
As I finally found a way to type the decorator (using mypy_extensions.NamedArg), I can finally make mypy aware of the methods' types.
This commit (as well as all three of the last commits) also fixes issues found by mypy thanks to this.
Depends on !506 (closed).
Migrated from D3759 (view on Phabricator)