Add multi-query support to label step

[porunov]

Benchmarks: #3915 (comment)

Fixes #3813

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[porunov]

Benchmarks

master branch:

Benchmark                         (fanoutFactor)  Mode  Cnt      Score     Error  Units
CQLMultiQueryBenchmark.getLabels             100  avgt    5    833.018 ±  34.454  ms/op
CQLMultiQueryBenchmark.getLabels             500  avgt    5  21106.121 ± 416.844  ms/op

This PR branch:

Benchmark                         (fanoutFactor)  Mode  Cnt     Score     Error  Units
CQLMultiQueryBenchmark.getLabels             100  avgt    5    70.820 ±   5.405  ms/op
CQLMultiQueryBenchmark.getLabels             500  avgt    5  1725.351 ± 102.009  ms/op

Conclusion:
In this PR we see label step performs about 12 times faster than in the master branch.

[FlorianHockmann]

LGTM, I only found one nit.

[li-boxuan]

Nice!