Align BatchWriteItem affinity query plan

[dkropachev]

Fixes #66

Jira: https://scylladb.atlassian.net/browse/DRIVER-658
Epic: https://scylladb.atlassian.net/browse/DRIVER-653

Summary:

• Add support for multiple preferred nodes in AlternatorQueryPlan while keeping single-node constructor usage working.
• Order BatchWriteItem voted nodes by vote count, then coordinator URL.
• Ignore malformed BatchWriteItem entries that contain both PutRequest and DeleteRequest.

Tests:

• npm run typecheck
• npm run lint
• npm run test

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.