cassandra: logged batch

[maxim-ge]

• https://docs.datastax.com/en/cql-oss/3.x/cql/cql_reference/cqlBatch.html
• https://docs.datastax.com/en/cql/dse/docs/reference/cql-commands/batch.html

  Although a logged batch enforces atomicity (that is, it guarantees if all DML statements in the batch succeed or none do), Cassandra does no other transactional enforcement at the batch level. For example, there is no batch isolation unless the batch operation is writing to a single partition. In multiple partition batch operations, clients are able to read the first updated rows from the batch, while other rows are still being updated on the server. In single partition batch operations, clients cannot read a partial update from any row until the batch is completed.