Put filewatcher in a worker, for better stability and performance

[DeMoorJasper]

This PR puts FSWatcher into a worker, this helps to add extra stability.

Fixes:

• Segfault error inside fsevents
• Freezing on windows
• Significant performance boost in watch mode

Some tests I've done on performance:

Description	With current master	with fswatcher-child
Very small app without cache (On Mac OS X High Sierra)	8.8secs	4secs
Small app with a lot of requires and import without cache (On windows 10)	22secs	7secs

fswatcher-child: https://github.com/DeMoorJasper/fswatcher-child

Closes #1342 Closes #1331 Closes #1394 Closes #1357 Closes #1308

[DeMoorJasper]

Tests pass now, only the Cargo one fails which is unrelated to this PR

[devongovett]

hmm interesting idea. what do you think about using our existing worker infra in parcel for this?

[DeMoorJasper]

@devongovett Thought it would be a better idea to make it lightweight.
The main goal was to sandbox chokidar from parcel to prevent errors and crashes. It doesn’t need multiple workers and localworker in this case wouldn’t prevent the crashes

_{Sent with GitHawk}

[DeMoorJasper]

@devongovett I've added some benchmarks, as I've just discovered after running the inspector on a project that it also boosts dev builds on non windows machines quite a lot