Use PARCEL_MAX_CONCURRENT_CALLS environment variable

[albinotonnina]

I had an issue with Parcel freezing during bundling.
@DeMoorJasper was so kind to help debugging the issue.
Turns out lowering the amount of concurrent calls per worker solves my issue.

This PR let users set that value with an env variable.

Thanks

[KeineLimonade]

I think the default value of 10 should still be used in case no environment variable is present.

[albinotonnina]

@KeineLimonade of course! silly me, fixed

[devongovett]

do we know what is causing the freeze? I'd rather fix that than make everyone configure this.

[DeMoorJasper]

This seems to happen only on projects that are fairly large and use scss, So my guess is it's scss related.

Although I haven't dug very deep into why exactly it is happening, in the example @albinotonnina gave me I could eliminate the freeze by removing all scss imports.

I've done some debugging on the workerfarm and processing 10 assets at the same time makes some assets take a very long time to process (more than 10secs). so I think a lower default would be a good idea, although it would probably have very little performance effect (besides from putting less stress on the workers)

[albinotonnina]

It would be an advanced setting, like the other env variable PARCEL_WORKERS.
I guess most people will rarely use it but it could be a good un-blocker while looking for a solution in cases such as mine. It surely makes me continue working!
Btw thanks again @DeMoorJasper. @devongovett you should take him with you at Adobe! :P

[Hammster]

@DeMoorJasper i can confirm that after removing all scss in my example it builds again too. https://github.com/Hammster/parcel-1.9.x-freeze

And @albinotonnina reasoning is totally valid, we are stuck using 1.7.1 because the build freezez, having this option would at least allow us to use the latest version.