Allow environment var GOMAXPROCS=<N> to override NumCPU().

[moodyjon]

Default value of GOMAXPROCS is NumCPU(), so ordinary behavior does not change.

However, this allows tuning the number of threads used for claimtrie rebuild.

[coveralls]

Pull Request Test Coverage Report for Build 2496946736

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.007%) to 51.823%

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Totals
Change from base Build 2450492318:	0.007%
Covered Lines:	22456
Relevant Lines:	43332

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💛 - Coveralls

[roylee17]

LGTM. Thanks