partially resolve #100 (bucket_stat)

[nurzhan-saktaganov]

• custom msgpackv5 decoders
• use StorageCallVShardError instead of bucketStatError

What has been done? Why? What problem is being solved?

I didn't forget about (remove if it is not applicable):

• Changelog (see documentation for changelog format)

Related issues:

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 54.54545% with 30 lines in your changes missing coverage. Please review.

Project coverage is 68.37%. Comparing base (a486c03) to head (300523f).

Files with missing lines	Patch %	Lines
vshard.go	42.30%	10 Missing and 5 partials ⚠️
replicaset.go	63.15%	10 Missing and 4 partials ⚠️
discovery.go	50.00%	0 Missing and 1 partial ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #113      +/-   ##
==========================================
+ Coverage   68.02%   68.37%   +0.35%     
==========================================
  Files          13       13              
  Lines        1301     1344      +43     
==========================================
+ Hits          885      919      +34     
- Misses        363      367       +4     
- Partials       53       58       +5     

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