Allow environment var GOGC=<percent> to override hard-coded SetGCPercent(10)

[moodyjon]

Unbundled from PR #43

See that for usage/testing. (TODO)

The default (10) only lets heap grow 10% before triggering GC. Running with (for example) "GOGC=100" will reduce CPU usage while heap is allowed to fluctuate more.

[coveralls]

Pull Request Test Coverage Report for Build 2434997880

• 0 of 3 (0.0%) changed or added relevant lines in 1 file are covered.
• 5 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.01%) to 51.793%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lbcd.go	0	3	0.0%

Files with Coverage Reduction	New Missed Lines	%
peer/peer.go	2	76.31%
btcec/signature.go	3	92.82%

Totals
Change from base Build 2403229780:	0.01%
Covered Lines:	22413
Relevant Lines:	43274

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💛 - Coveralls

[roylee17]

LGTM. Thanks!