fix: handle database switch correctly

[jjeffcaii]

What this PR does:

Will handle use <db> correctly.

Which issue(s) this PR fixes:

Fixes #595

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[codecov-commenter]

Codecov Report

Base: 36.89% // Head: 37.34% // Increases project coverage by +0.45% 🎉

Coverage data is based on head (d763c7e) compared to base (4bbc3f4).
Patch coverage: 14.52% of modified lines in pull request are covered.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #618      +/-   ##
==========================================
+ Coverage   36.89%   37.34%   +0.45%     
==========================================
  Files         198      198              
  Lines       22141    22773     +632     
==========================================
+ Hits         8168     8505     +337     
- Misses      13065    13338     +273     
- Partials      908      930      +22     

Impacted Files	Coverage Δ
pkg/constants/mysql/constants.go	100.00% <ø> (ø)
pkg/mysql/execute_handle.go	0.00% <0.00%> (ø)
pkg/mysql/server.go	0.00% <0.00%> (ø)
pkg/runtime/function/cast_date.go	88.23% <ø> (+0.23%)	⬆️
pkg/runtime/function/cast_time.go	87.61% <ø> (+2.41%)	⬆️
pkg/executor/redirect.go	5.53% <5.00%> (+1.45%)	⬆️
pkg/mysql/conn.go	46.92% <16.66%> (+9.75%)	⬆️
pkg/mysql/client.go	30.73% <50.00%> (ø)
pkg/boot/discovery.go	58.29% <100.00%> (+10.88%)	⬆️
pkg/runtime/function/cast_datetime.go	95.81% <100.00%> (+7.18%)	⬆️
... and 11 more

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