schema-agreement: add session fallback

[dkropachev]

Summary:

• add Session.wait_for_schema_agreement() that polls local session hosts via system.local
• fall back from control-connection schema agreement to a live session when the control connection is unavailable or closed
• fall back to the session API after DDL schema-refresh agreement fails

Fixes #604

Testing:

• python -m py_compile cassandra/cluster.py tests/unit/test_control_connection.py tests/unit/test_cluster.py tests/unit/test_response_future.py
• direct probes for modified control/response-future tests
• repo_ci fast: workflow-build, workflow-prepare, and workflow-test passed; workflow-build-2 timed out after 300s in cibuildwheel

[Lorak-mmk]

fall back to the session API after DDL schema-refresh agreement fails

I don't think this is correct. The purpose of after-DDL schema agreement is to make sure that all the nodes acknowledged the new schema notification.
If you perform the schema agreement check on a different node, it is possible that it is the node that didn't get the schema change yet, and will get a false-positive schema agreement.

After-DDL schema agreement should be performed on the node we used for DDL. We can use different connection, but not different node.

[dkropachev]

fall back to the session API after DDL schema-refresh agreement fails

I don't think this is correct. The purpose of after-DDL schema agreement is to make sure that all the nodes acknowledged the new schema notification. If you perform the schema agreement check on a different node, it is possible that it is the node that didn't get the schema change yet, and will get a false-positive schema agreement.

After-DDL schema agreement should be performed on the node we used for DDL. We can use different connection, but not different node.

It does it on all active nodes.