fix: another config deadlock by trying to acquire a read lock twice

[atzoum]

Description

config#registerXxxVar was acquiring a read lock on vLock and after that, another read lock was being acquired on the same lock by config#GetXxx before the previous read lock was released, causing a deadlock.

• Simplifying locking by only acquiring locks when necessary (limited scope) and once
• Adding a test to protect against a future regression
• Avoiding to register a reloadable pointer more than once

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

Patch coverage: 100.00% and project coverage change: -0.05% ⚠️

Comparison is base (ac71452) 79.28% compared to head (72266ee) 79.24%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #120      +/-   ##
==========================================
- Coverage   79.28%   79.24%   -0.05%     
==========================================
  Files          68       68              
  Lines        5089     5079      -10     
==========================================
- Hits         4035     4025      -10     
  Misses        853      853              
  Partials      201      201              

Files Changed	Coverage Δ
config/config.go	82.79% <100.00%> (ø)
config/hotreloadable.go	87.36% <100.00%> (-0.27%)	⬇️
config/load.go	92.17% <100.00%> (-0.09%)	⬇️

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[fracasula]

Nice one Aris! We've been improving this lib quite a bit 💪