fs: do not emit 'stop' watch event synchronously

Emits 'stop' event for fs.watchFile on process.nextTick
to fix 'maximum call stack size exceeded' error when
`stop` is called synchronously after listener is attached.

PR-URL: https://github.com/nodejs/node/pull/8524
Fixes: https://github.com/nodejs/node/issues/8421


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Yorkie Liu <yorkiefixer@gmail.com>