Prevent stale frontend responses from replacing newer selections

[gordonmurray]

Closes #79.

Summary

• assign monotonically increasing request IDs to dataset-list, metadata, and row requests
• ignore successful or failed responses when a newer request has started
• capture the selected dataset when metadata and row requests begin
• invalidate in-flight metadata and row requests when the connection changes
• add dependency-free Node tests that complete requests in reverse order
• run the frontend syntax and race tests as a required CI job

Covered races

• an older dataset-list response after a newer connection
• rows from an older dataset after a newer dataset selection
• rows from an older column selection on the same dataset
• metadata from an older dataset
• an older failed request after a newer request succeeds

Validation

• node --check web/vanilla/app.js
• node --test web/vanilla/tests/*.test.js — 5 passed
• ../testing/run-tests.sh 0.36.0 — 65 passed
• built the LanceDB 0.36.0 application image
• smoke-tested /healthz, /datasets, /, and /app.js