[Objective-C] Thread safety fix for Object class

[michalcichon]

Synchronizing on [self class] makes several critical sections across all inheriting classes with one shared NSDictionary object (classNames). I believe it wasn't the intention of the author of this fix (based on a JSONModel issue ticket). To make one critical section we need one static object (lock).

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
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Description of the PR

Synchronizing on [self class] makes several critical sections across all inheriting classes with one shared NSDictionary object (classNames). I believe it wasn't the intention of the author of this fix (based on a JSONModel issue ticket). To make one critical section we need one static object (lock).

[michalcichon]

Unfortunately there is no technical committee for Objective-C. 😞

[webron]

@michalcichon - do you want to be in one? :)

[michalcichon]

Hey @webron! I haven't got 3 PRs merged, so I'm afraid I'm not eligible. 😉 My concern was about who's gonna review my pull request. But if you like you can add me as an "interim" member of the technical committee just in case if someone has the same problem as I have.