http: refactor outgoing headers processing

[apapirovski]

Use a shared function, for..in instead of Object.keys, do less work in
setHeader and instead defer some of it until later, and other minor
changes to improve clarity, as well as a slight boost in performance.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

[apapirovski]

CI: https://ci.nodejs.org/job/node-test-pull-request/14472/
CitGM: https://ci.nodejs.org/view/Node.js-citgm/job/citgm-smoker/1386/
Benchmark CI:
https://ci.nodejs.org/view/Node.js%20benchmark/job/benchmark-node-micro-benchmarks/164/
https://ci.nodejs.org/view/Node.js%20benchmark/job/benchmark-node-micro-benchmarks/165/
https://ci.nodejs.org/view/Node.js%20benchmark/job/benchmark-node-micro-benchmarks/166/

[apapirovski]

Benchmarks:

http/headers.js n=1000 duplicates=100 benchmarker='wrk'        ***     16.28 %       ±2.83% ±3.78% ±4.94%
http/headers.js n=1000 duplicates=1 benchmarker='wrk'          ***     -3.92 %       ±1.59% ±2.11% ±2.75%
http/headers.js n=10 duplicates=100 benchmarker='wrk'          ***     14.42 %       ±5.04% ±6.71% ±8.73%
http/headers.js n=10 duplicates=1 benchmarker='wrk'                     1.70 %       ±3.82% ±5.09% ±6.62%

http/set_header.js n=1000000 value='Connection'               ***      8.50 %       ±2.42% ±3.20% ±4.11%
http/set_header.js n=1000000 value='Content-Length'           ***      5.65 %       ±2.55% ±3.36% ±4.32%
http/set_header.js n=1000000 value='Content-Type'             ***      5.57 %       ±2.51% ±3.31% ±4.25%
http/set_header.js n=1000000 value='Set-Cookie'               ***      6.51 %       ±1.98% ±2.62% ±3.36%
http/set_header.js n=1000000 value='Transfer-Encoding'        ***      7.07 %       ±1.88% ±2.48% ±3.19%
http/set_header.js n=1000000 value='Vary'                     ***      9.03 %       ±2.46% ±3.24% ±4.17%
http/set_header.js n=1000000 value='X-Powered-By'             ***      5.24 %       ±2.31% ±3.04% ±3.91%

http/set-header.js res='normal' benchmarker='wrk'                      0.49 %       ±2.12% ±2.80% ±3.60%
http/set-header.js res='setHeader' benchmarker='wrk'                   0.22 %       ±2.30% ±3.03% ±3.89%
http/set-header.js res='setHeaderWH' benchmarker='wrk'                 0.48 %       ±1.94% ±2.56% ±3.28%

[trivikr]

CI rerun for node-test-commit-osx https://ci.nodejs.org/job/node-test-commit-osx/18148/

[addaleax]

Landed in 602ffd6