fix(warehouse): use correct config for columns batch size

[achettyiitr]

Description

• We are using the wrong config for columnsBatchSize as customDatasetPrefix.
• Also we don't need to log the load files information for BiqQuery. It is polluting the logs.

Notion Ticket

https://www.notion.so/rudderstacks/Use-batching-to-run-several-of-alter-statements-81ae26d5db6f4b2ca8e0258e3ce0cba6

Security

• The code changed/added as part of this pull request won't create any security issues with how the software is being used.

[codecov]

Codecov Report

Base: 45.85% // Head: 45.86% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (ae5b931) compared to base (712b6a5).
Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2720      +/-   ##
==========================================
+ Coverage   45.85%   45.86%   +0.01%     
==========================================
  Files         295      295              
  Lines       48499    48499              
==========================================
+ Hits        22239    22244       +5     
+ Misses      24857    24853       -4     
+ Partials     1403     1402       -1     

Impacted Files	Coverage Δ
warehouse/bigquery/bigquery.go	1.12% <0.00%> (ø)
warehouse/upload.go	18.45% <0.00%> (ø)
config/backend-config/namespace_config.go	67.00% <0.00%> (-3.00%)	⬇️
processor/processor.go	86.40% <0.00%> (+0.41%)	⬆️

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