fix: mod zero

[chunshao90]

Which issue does this PR close?

Closes #

Rationale for this change

Fix the % 0 bug that may occur during the cluster initialization phase.

What changes are included in this PR?

• Fix the % 0 bug that may occur during the cluster initialization phase.
• Refactor PickShards function.
• Pick shard to random.

Are there any user-facing changes?

No.

How does this change test

CI.

[codecov-commenter]

Codecov Report

Merging #171 (e231110) into main (7f1354e) will decrease coverage by 0.08%.
The diff coverage is 74.19%.

❗ Current head e231110 differs from pull request most recent head 0d8b7a4. Consider uploading reports for the commit 0d8b7a4 to get more accurate results

@@            Coverage Diff             @@
##             main     #171      +/-   ##
==========================================
- Coverage   39.66%   39.59%   -0.08%     
==========================================
  Files          34       34              
  Lines        4213     4213              
==========================================
- Hits         1671     1668       -3     
- Misses       2317     2319       +2     
- Partials      225      226       +1     

Impacted Files	Coverage Δ
server/coordinator/factory.go	4.45% <0.00%> (ø)
server/coordinator/shard_picker.go	88.00% <79.31%> (-6.00%)	⬇️

[ZuLiangWang]

LGTM