Dynamically resolve rack.multithread/rack.multiprocess

[FTLam11]

Closes #2174.

This PR dynamically resolves rack.multithread and rack.multiprocess.

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[nateberkopec]

Thanks for your contribution. Ideally, the user should not have to add any config settings. We already know if their app multithread if the threadpool max size is > 1, and we know it's multiprocess if workers >= 1. So the extra config settings aren't necessary here IMO.

[nateberkopec]

Also, the values that need to change in response to config are here:

puma/lib/puma/binder.rb

Line 26 in b046d60

"rack.multithread".freeze => true,

The problem is that these values are frozen and constant.

[FTLam11]

@nateberkopec Appreciate the explanation/hints, thank you!

[nateberkopec]

Quick change then LGTM