DAX does not support control plane operations like DescribeTable, CreateTable,
or DeleteTable. These must use the regular DynamoDB client regardless of
whether DAX is enabled for data operations.

This fix ensures:
- update() method uses regular DynamoDB for table existence checks and creation
- teardown() method uses regular DynamoDB for table deletion
- online_read() and online_write_batch() continue to use DAX for data ops

The _get_dynamodb_client/resource methods cache the first client created.
When update() creates a non-DAX client first, it gets cached and reused
by online_read() even when DAX is enabled.

Fix: Use _initialize_dynamodb_* directly in update() and teardown() to
create fresh non-cached clients, allowing the cached versions to be
created by data operations (online_read/online_write) with DAX enabled.