Add push-down predicate in CassandraIO

Now that `CassandraIO` has been merged, I will add a new feature: support of user query to filter at the IO level (push-down predicate).

Imported from Jira [BEAM-2420](https://issues.apache.org/jira/browse/BEAM-2420). Original Jira may contain additional context.
Reported by: jbonofre.

Start by reading the CassandraIO implementation and the additional context in BEAM-2420. Determine how a user query should be represented and where filtering can occur at the IO level; done means CassandraIO supports predicate push-down with coverage for the intended behavior.

Written by the indexing model from the issue text.