refactor(cubesql): Fix clippy warnings in cubesql, part 1

[mcheshkov]

Check List

• Tests has been run in packages where changes made if available
• Linter has been run for changed code
• Tests for the changes have been added if not covered yet
• Docs have been added / updated if required

Description of Changes Made (if issue reference is not provided)

Just a bunch of small clippy fixes around the codebase, targeted to enable more rules

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 11 lines in your changes missing coverage. Please review.

Project coverage is 82.85%. Comparing base (10e47fc) to head (515d19c).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
rust/cubesql/pg-srv/src/buffer.rs	64.70%	6 Missing ⚠️
...t/cubesql/cubesql/src/compile/engine/udf/common.rs	80.00%	2 Missing ⚠️
rust/cubesql/cubesql/src/compile/router.rs	75.00%	1 Missing ⚠️
rust/cubesql/cubesql/src/sql/postgres/shim.rs	83.33%	1 Missing ⚠️
rust/cubesql/cubesql/src/sql/statement.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #9052   +/-   ##
=======================================
  Coverage   82.85%   82.85%           
=======================================
  Files         221      221           
  Lines       78813    78791   -22     
=======================================
- Hits        65300    65284   -16     
+ Misses      13513    13507    -6     

Flag	Coverage Δ
cubesql	82.85% <85.71%> (+<0.01%)	⬆️

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