Get rid of `pycassa`

[Hyask]

Context:

• Have a look at src/errors/cassie.py. It's importing pycassa, which is an old Python module that I want to get rid of.
• The new API uses an ORM-like mechanism provided by the cassandra module.
• You can find the cassandra module here: https://github.com/apache/cassandra-python-driver
• You can find already defined models in src/errortracker/cassandra_schema.py.

Tasks:

• Add any missing model found in src/errors/cassie.py to src/errortracker/cassandra_schema.py.
• Replace all the calls to pycassa in src/errors/cassie.py by queries using the models in src/errortracker/cassandra_schema.py.