core: add buckets property to regionInfo struct

[bufferflies]

Signed-off-by: bufferflies 1045931706@qq.com

What problem does this PR solve?

This is the first step for the final storage of buckets in memery.
Some api and storage depences on it.
Issue Number: Relate #4668

What is changed and how it works?

Check List

Tests

Code changes

Side effects

Related changes

Release note

 None.

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[codecov]

Codecov Report

Merging #4743 (e5dc99d) into master (950b363) will increase coverage by 0.04%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #4743      +/-   ##
==========================================
+ Coverage   74.99%   75.04%   +0.04%     
==========================================
  Files         287      287              
  Lines       28140    28145       +5     
==========================================
+ Hits        21104    21121      +17     
+ Misses       5158     5146      -12     
  Partials     1878     1878              

Flag	Coverage Δ
unittests	75.04% <0.00%> (+0.04%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
server/core/region.go	91.40% <0.00%> (-0.19%)	⬇️
server/core/region_option.go	80.95% <0.00%> (-1.58%)	⬇️
server/region_syncer/client.go	83.59% <0.00%> (-1.57%)	⬇️
server/schedule/operator/step.go	74.07% <0.00%> (-1.02%)	⬇️
server/cluster/cluster.go	83.52% <0.00%> (-0.52%)	⬇️
server/schedulers/hot_region.go	83.08% <0.00%> (-0.31%)	⬇️
client/client.go	68.72% <0.00%> (-0.11%)	⬇️
pkg/tempurl/tempurl.go	60.00% <0.00%> (ø)
server/schedulers/balance_leader.go	92.36% <0.00%> (+0.06%)	⬆️
server/server.go	75.07% <0.00%> (+0.15%)	⬆️
... and 8 more

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[rleungx]

When will it happen?

[bufferflies]

this is the first step, sync regions and api related region will use it, I will split them into different pr. like this:#4699 and pingcap/kvproto#867.

[rleungx]

I mean r == nil

[ti-chi-bot]

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Notice: Please provide the linked issue number on one line in the PR body, for example: Issue Number: close #123 or Issue Number: ref #456, multiple issues should use full syntax for each issue and be separated by a comma, like: Issue Number: close #123, ref #456.

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[bufferflies]

/close see #4670