Rename db Engines related functions

[lunny]

To make it not confusing, this PR renamed some functions related database Engine.

• GetNewEngine -> NewEngine: this function will return a xorm.Engine as the first return parameter, so call it New is suitable.
• SetEngine -> InitEngine: it will call the original GetNewEngine and assign engine to the db package variable x. So we call it Init.
• NewEngine -> InitEngineWithMigration: the function will call SetEngine with some migration functions.
• NewInstallTestEngine -> InitInstallEngineWithMigration: the function will call SetEngine with some test special statements.

[codecov-commenter]

Codecov Report

Merging #17481 (b305ba2) into main (0b4a8be) will increase coverage by 0.00%.
The diff coverage is 19.23%.

@@           Coverage Diff           @@
##             main   #17481   +/-   ##
=======================================
  Coverage   45.47%   45.47%           
=======================================
  Files         791      791           
  Lines       88697    88702    +5     
=======================================
+ Hits        40335    40339    +4     
+ Misses      41850    41845    -5     
- Partials     6512     6518    +6     

Impacted Files	Coverage Δ
cmd/cmd.go	44.18% <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)
modules/doctor/dbconsistency.go	6.83% <0.00%> (ø)
modules/doctor/dbversion.go	42.85% <0.00%> (ø)
modules/doctor/doctor.go	12.24% <0.00%> (ø)
routers/install/install.go	0.66% <0.00%> (ø)
routers/web/feed/convert.go	0.00% <0.00%> (ø)
... and 12 more

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[wxiaoguang]

I like the idea to make these function names clear.