refactor: remove DB generic

[bsbds]

Please briefly answer these questions:

• what problem are you trying to solve? (or if there's no problem, what's the motivation for this change?)

  The DB generic is redundant as no other type will ever implement StorageApi in Xline. This change aims to enhance readability of the codebase.

• what changes does this pull request make?

• are there any non-obvious implications of these changes? (does it break compatibility with previous versions, etc)

[codecov]

Codecov Report

Attention: Patch coverage is 86.32479% with 16 lines in your changes missing coverage. Please review.

Project coverage is 75.70%. Comparing base (e35b35a) to head (d836804).
Report is 117 commits behind head on master.

Files	Patch %	Lines
crates/xline/src/server/command.rs	64.70%	3 Missing and 3 partials ⚠️
crates/xline/src/server/maintenance.rs	62.50%	1 Missing and 5 partials ⚠️
crates/xline/src/server/xline_server.rs	89.65%	0 Missing and 3 partials ⚠️
crates/xline/src/storage/db.rs	94.73%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #806      +/-   ##
==========================================
+ Coverage   75.55%   75.70%   +0.15%     
==========================================
  Files         180      187       +7     
  Lines       26938    27822     +884     
  Branches    26938    27822     +884     
==========================================
+ Hits        20353    21063     +710     
- Misses       5366     5470     +104     
- Partials     1219     1289      +70     

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[kamyuentse]

Though xline currently has only one StorageApi impl，the generic is make sense for the community if someone want to use another storage engine.

[bsbds]

Though xline currently has only one StorageApi impl，the generic is make sense for the community if someone want to use another storage engine.

The engine crate already provides a level of abstraction, I think introducing the StorageApi would be redundant.

[rogercloud]

If we remove the DB generic here, is it necessary to keep StorageApi trait?

[bsbds]

If we remove the DB generic here, is it necessary to keep StorageApi trait?

removed

[Phoenix500526]

LGTM. Please squash these commits into one commit

[CrystalAnalyst]

LGTM.