chore: warehouse tests race detection

[fracasula]

Description

Running warehouse tests with -race.

Changelog

• running warehouse tests with -race in order to be able to detect bugs due unregulated concurrent access to the same variable(s)
• added a way to clone model.Schema because even if we use a mutex to protect the concurrent access, we're still passing around a reference so a change to the map would still count as a data race
• moved Schema into its own package that now has both a public and a private API
  • this was done to decouple and to make sure all concurrent accesses are handled properly since the mutexes are now encapsulated in the type
  • moved public API (exported methods) on top
• moved uploadSchema into UploadJob
• moved handleSchemaChange and its errors to slave_worker since it's specific to that business logic

Linear Ticket

< Linear Link >

Security

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

[linear]

PIPE-119 Run Warehouse tests with -race

We should change our continous integration pipeline to run the warehouse tests with -race.

Any data race detected is going to make the tests fail even if all assertions pass so resulting races should be eliminated as well.

[codecov]

Codecov Report

Patch coverage: 93.65% and project coverage change: +0.03% 🎉

Comparison is base (890ccb6) 68.98% compared to head (869be23) 69.02%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3773      +/-   ##
==========================================
+ Coverage   68.98%   69.02%   +0.03%     
==========================================
  Files         348      349       +1     
  Lines       51834    51886      +52     
==========================================
+ Hits        35760    35815      +55     
+ Misses      13772    13770       -2     
+ Partials     2302     2301       -1     

Files Changed	Coverage Δ
warehouse/router_identities.go	19.83% <0.00%> (ø)
warehouse/schema/schema.go	96.00% <92.59%> (ø)
warehouse/upload.go	71.90% <92.98%> (+0.38%)	⬆️
warehouse/internal/model/schema.go	100.00% <100.00%> (ø)
warehouse/slave_worker.go	83.50% <100.00%> (+1.86%)	⬆️

... and 7 files with indirect coverage changes

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[fracasula]

This will go away anyway due to this PR.