[C] Port the batched version of MultiGet() to RocksDB's C API

[yhchiang-sol]

Summary:
The batched version of MultiGet() is not available in RocksDB's C API.
This PR implements rocksdb_batched_multi_get_cf which is a C wrapper function
that invokes the batched version of MultiGet() which takes one single column family.

Test Plan:
Added a new test case under "columnfamilies" test case in c_test.cc

[yhchiang-sol]

The error on Travis CI - Pull Request seems transient and not related to this PR.

The command "sudo -E apt-add-repository -y "ppa:ubuntu-toolchain-r/test"" failed and exited with 1 during .

[yhchiang-sol]

Hello, @siying, @ajkr. This is YH :p.

We want to use the batched version of MultiGet in our use case, but this PR is a blocker because the function is not available in the C API.

Do you have time to take a look or happen to know who might be the best person to review?
Thank you!

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@ajkr has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[ajkr]

Hi YH :), thanks for the PR. LGTM!

[yhchiang-sol]

Thank you, @ajkr. Should I rebase or it's good to merge?

[ajkr]

Thank you, @ajkr. Should I rebase or it's good to merge?

Should be good to merge - just waiting for someone to accept the internal diff.

[yhchiang-sol]

Should be good to merge - just waiting for someone to accept the internal diff.

Thank you. Let me know if there's anything that I should address.

[yhchiang-sol]

Hello, @ajkr. Can I know whether the PR looks good internally?