[vzaidman]

Summary

Pre-build the 3 most recently used bundles when Metro starts serving, most recent first, so the first request after a restart is served from a warm build.

• A request for a bundle being pre-built joins that build instead of starting a second one.
• Any bundle request cancels the remaining pre-warm queue; an in-flight pre-build keeps going and serves the request.
• Recency is recorded only for successfully served bundle requests and persists across restarts.

Changelog: [Performance] Pre-warm the 3 most recently used bundles on server start

Test plan

Added packages/metro/src/Server/__tests__/Server-prewarm-test.js, covering: pre-building the 3 most recent bundles on start, sharing an in-flight pre-build with a concurrent request, cancelling queued pre-warms on a bundle request, taking over a queued pre-warmed bundle, surviving a failed pre-warm build, and persisting recency across restarts with only successful requests updating it.

$ yarn jest packages/metro/src/Server
Test Suites: 4 passed, 4 total
Tests:       111 passed, 111 total