Use a semaphore instead of WaitGroup for FlatMap parallelism

[teivah]

Based on @dparrish's PR: #168

[coveralls]

Pull Request Test Coverage Report for Build 363

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.1%) to 85.759%

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Files with Coverage Reduction	New Missed Lines	%
observer.go	1	89.72%

Totals
Change from base Build 355:	-0.1%
Covered Lines:	1927
Relevant Lines:	2247

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 363

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.1%) to 85.759%

---

Files with Coverage Reduction	New Missed Lines	%
observer.go	1	89.72%

Totals
Change from base Build 355:	-0.1%
Covered Lines:	1927
Relevant Lines:	2247

---

💛 - Coveralls

[teivah]

Thanks @dparrish