Add a low priority to the scheduler that still resolves

[n8schloss]

Normal priority is for things that we still want the user to see and idle pri is for things that are more speculative and may never resolve. We do not currently have a priority for things (like logging or sending data back to servers) where we want to ensure that it resolves, but where we want other things to be able to happen first. This adds such a priority.

I was debating a bit between calling it 'low' priority or 'logging' priority. I went with calling it 'low' because you can use this for way more than logging, but opening to changing it.

[sizebot]

Details of bundled changes.

Comparing: 051272f...da88002

scheduler

File	Filesize Diff	Gzip Diff	Prev Size	Current Size	Prev Gzip	Current Gzip	ENV
scheduler.development.js	n/a	n/a	0 B	19.17 KB	0 B	5.74 KB	UMD_DEV
scheduler.production.min.js	n/a	n/a	0 B	3.16 KB	0 B	1.53 KB	UMD_PROD
scheduler.development.js	+1.0%	+0.7%	21.56 KB	21.77 KB	5.84 KB	5.88 KB	NODE_DEV
scheduler.production.min.js	🔺+1.2%	🔺+1.1%	4.64 KB	4.7 KB	1.82 KB	1.84 KB	NODE_PROD
Scheduler-dev.js	+1.0%	+0.6%	21.79 KB	22 KB	5.89 KB	5.92 KB	FB_WWW_DEV
Scheduler-prod.js	🔺+0.9%	🔺+0.6%	13.06 KB	13.18 KB	2.86 KB	2.88 KB	FB_WWW_PROD

Generated by 🚫 dangerJS

[sebmarkbage]

Seems reasonable to me. Thoughts, @acdlite?

[acdlite]

How about idle priority? :)

[acdlite]

Oh duh, we already that one. I thought it was still "offscreen" or something.

Let's merge and figure out the name later.