Prevent data loss when updating materialized view (#383)

[the4thamigo-uk]

Summary

Dropping and recreating a materialized view risk losing some data. Instead we should use ALTER TABLE...MODIFY QUERY which performs the update atomically.

Checklist

Delete items not relevant to your PR:

• Unit and integration tests covering the common scenarios were added
• [X ] A human-readable description of the changes was provided to include in CHANGELOG
• For significant changes, documentation in https://github.com/ClickHouse/clickhouse-docs was updated with further explanations or tutorials