api: prevent store limit from changing to zero (#2586)

[ti-srebot]

cherry-pick #2586 to release-4.0

---

What problem does this PR solve?

Closes #2584.

What is changed and how it works?

When setting the store limit, we first check if it is zero and reject the request if it is.

Check List

Tests

• Unit test

Related changes

• Need to cherry-pick to the release branch

Release note

• Prevent store limit from changing to zero

[codecov-commenter]

Codecov Report

Merging #2588 into release-4.0 will increase coverage by 0.07%.
The diff coverage is 50.00%.

@@               Coverage Diff               @@
##           release-4.0    #2588      +/-   ##
===============================================
+ Coverage        77.13%   77.21%   +0.07%     
===============================================
  Files              205      205              
  Lines            22129    22129              
===============================================
+ Hits             17070    17086      +16     
+ Misses            3756     3742      -14     
+ Partials          1303     1301       -2     

Impacted Files	Coverage Δ
server/api/store.go	66.32% <50.00%> (+1.02%)	⬆️
server/kv/etcd_kv.go	84.41% <0.00%> (-3.90%)	⬇️
pkg/dashboard/adapter/manager.go	89.62% <0.00%> (-3.78%)	⬇️
server/cluster/coordinator.go	74.52% <0.00%> (-2.20%)	⬇️
server/handler.go	50.53% <0.00%> (-0.44%)	⬇️
server/server.go	76.48% <0.00%> (-0.31%)	⬇️
pkg/btree/btree.go	87.65% <0.00%> (+0.80%)	⬆️
pkg/etcdutil/etcdutil.go	84.05% <0.00%> (+1.44%)	⬆️
pkg/mock/mockhbstream/mockhbstream.go	90.76% <0.00%> (+1.53%)	⬆️
server/tso/tso.go	83.21% <0.00%> (+2.18%)	⬆️
... and 3 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0bd6bbd...df9f972. Read the comment docs.

[HunDunDM]

LGTM

[nolouch]

/merge

[ti-srebot]

Sorry @nolouch, you don't have permission to trigger auto merge event on this branch.
The version releasement is in progress.

[disksing]

/merge

[ti-srebot]

Sorry @disksing, you don't have permission to trigger auto merge event on this branch.
The version releasement is in progress.

[HunDunDM]

/merge

[ti-srebot]

Your auto merge job has been accepted, waiting for:

• 2565

[ti-srebot]

/run-all-tests