Implemented thread safe storage of SessionManager for Alamofire

[JohannesNevels]

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

Fixed random cancelling Alamofire requests due to saving the references in a not thread safe dictionary. This occurs when doing multiple request at the same time. One or multiple requests are deallocated / cancelled when references are not correctly stored.