Move db related basic functions to models/db

[lunny]

Since models becomes more and more big, splitting it becomes a task to refactor.

This PR create a new subpackage models/db and move common database functions to that package.

The next PRs should create business logic related subpackages of models and move related codes there. i.e. Moving all models/issue*.go to models/issues/.

[KN4CK3R]

Otherwise lgtm

[codecov-commenter]

Codecov Report

Merging #17075 (d541431) into main (6532aa2) will increase coverage by 0.01%.
The diff coverage is 68.75%.

@@            Coverage Diff             @@
##             main   #17075      +/-   ##
==========================================
+ Coverage   45.24%   45.25%   +0.01%     
==========================================
  Files         771      773       +2     
  Lines       86767    86828      +61     
==========================================
+ Hits        39255    39292      +37     
- Misses      41160    41181      +21     
- Partials     6352     6355       +3     

Impacted Files	Coverage Δ
cmd/cmd.go	44.18% <0.00%> (ø)
cmd/convert.go	0.00% <0.00%> (ø)
cmd/doctor.go	0.00% <0.00%> (ø)
cmd/dump.go	0.91% <0.00%> (ø)
cmd/migrate.go	0.00% <0.00%> (ø)
cmd/migrate_storage.go	0.00% <0.00%> (ø)
models/db/convert.go	0.00% <ø> (ø)
models/db/log.go	31.70% <ø> (ø)
models/db/sequence.go	0.00% <0.00%> (ø)
models/db/sql_postgres_with_schema.go	0.00% <ø> (ø)
... and 178 more

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