performance for get shards index

[linger1216]

because numShards is power of two, so can use & operator for performance

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This change is 

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[linger1216]

merging is blocked , reason is

Function call made to an unsafe package
Found in z/bbloom.go and z/rtutil.go

It has nothing to do with this merge
 

[jarifibrahim]

Hey @linger1216 , the PR doesn't have benchmarks to support the performance improvement claim. Could you please point me to some references about % and & operator performance? A quick google search didn't give me anything useful.

Also, the merging is blocked because you haven't signed the CLA.