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.