Provide metrics for schema changes in Cassandra connector

Relational database history exposes a set of metrics to allow user to see how many schema changes are applied. Cassandra schema change listener should do the same albeit probably different set.

Also as a part of this PR the sleep calls introduced in https://github.com/debezium/debezium-connector-cassandra/pull/64 should be replaced with Awaitility waits chekcing the metrics change.