Document webpack change: Performance for caching

[webpack-bot]

A pull request by @sokra was merged and maintainers requested a documentation change.

See pull request: webpack/webpack#8651

---

• log a warning when serialization fails
• make reading from file safer and handle unexpected end of file
• update webpack-sources for faster buffer() call
• • see below

What kind of change does this PR introduce?
performance, bugfix

Did you add tests for your changes?
existing tests

Does this PR introduce a breaking change?
yes

What needs to be documented once your changes are merged?

• added cache.idleTimeout: Time in ms after which idle period the cache storing should happen (only for store: 'pack' or 'idle')
  • defaults to 10s
• added cache.idleTimeoutForInitialStore: Time in ms after which idle period the initial cache storing should happen (only for store: 'pack' or 'idle')
  • defaults to 0s
• removed Cache.hooks.got
• added argument to Cache.hooks.get gotHandlers: ((result, callback) => void)[]
• Compilation.file/context/missingDependenciesis now aSetinstead of aSortableSet` and it's no longer sorted.
• Module.libIdent(options) has no an optional options.associatedObjectForCache for caching
• FileCachePlugin idle actions do no longer keep the process running.

[bhavya9107]

@sokra where can I get started