Deprecate Cassandra

Cassandra backend is by far the least popular and it is lagging behind. We looked at it and determined that it's not feasible to provide config backend implementation based on Cassandra.

For those two reasons, we're going to deprecate Cassandra.

Cassandra will remain available in 2.x, but will be marked as deprecated. We will keep the code around as long as there are customers who still use it.