fix: concurrency

[xBlaz3kx]

Proposed changes

• Fixed read and write locking for concurrent operations

Types of changes

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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Checklist

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[github-actions]

Merging this branch will increase overall coverage

Impacted Packages	Coverage Δ	🤖
github.com/ChargePi/ocpp-manager/ocpp_v16	82.26% (+0.90%)	👍

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Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/ChargePi/ocpp-manager/ocpp_v16/manager.go	66.67% (+3.51%)	63 (+6)	42 (+6)	21	👍

Please note that the "Total", "Covered", and "Missed" counts above refer to code statements instead of lines of code. The value in brackets refers to the test coverage of that file in the old version of the code.