Fix logfile tailing under Node 10.16.0

[wearhere]

fs.watch does not consistently emit 'change' events on Node 10.16.0,
possibly due to nodejs/node#29460. fs.watchFile
still works, but is less efficient, per the docs, so fall back to that only if necessary and also suggest that users run custody under an older version of Node.

I explored several Node file-watching libraries (e.g. chokidar, sane) to try to
find a more performant solution for Node 10.16.0 and it seems that they pretty
much all fall back to fs.watch / fs.watchFile, unfortunately. sane offers
the option to use watchman, but that requires installing a daemon so is a
non-starter.

Fixes #82.

#83 tracks reverting this when the underlying Node issue has been fixed.

Tested that log files update both when running custody under v8.9.3 and
v10.16.0, and that node-tail only uses fs.watchFile in the latter.

[meatwallace]

Saw you merged, but "lgtm" anyway! I checked the issues and saw you're tracking the Node.js issue so we can revert - fantastic.

Barely related/relevant tidbit: not suggesting adding watchman as a dependency is a good idea unless you have a good reason, but the latest version of Flow has an opt-in setting for using watchman for file watching - If we converge to a monorepo at any point, we'll likely end up adopting it 😅

[wearhere]

the latest version of Flow has an opt-in setting for using watchman for file watching - If we converge to a monorepo at any point, we'll likely end up adopting it 😅

Oh neat. Yeah custody could switch to a version of node-tail that used sane to abstract over multiple file-watching libraries, and expose a flag like --useWatchman to let users that did have the daemon installed opt into that method.