feat(http): gzip compress the query CSV response.

[goller]

If client requests with Accept-Encoding: gzip, then
compress response and return with Content-Encoding: gzip.

This increases the server-side load by about 12%, but saves network bandwidth.

benchmark                     old ns/op     new ns/op     delta
Benchmark_Query_no_gzip-4     123609        137885        +11.55%

benchmark                     old allocs     new allocs     delta
Benchmark_Query_no_gzip-4     149            150            +0.67%

benchmark                     old bytes     new bytes     delta
Benchmark_Query_no_gzip-4     14297         15205         +6.35%

• CHANGELOG.md updated with a link to the PR (not the Issue)
• Well-formatted commit messages
• Rebased/mergeable
• Tests pass
• http/swagger.yml updated (if modified Go structs or API)
• Documentation updated or issue created (provide link to issue/pr)
• Signed CLA (if not already signed)

[chnn]

I'm wonder if nginx could compress more efficiently.