PR checklist

• [ x ] Read the contribution guildelines.
• [ x ] Ran the shell/batch script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run./bin/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates)
• [ x ] Filed the PR against the correct branch: master for non-breaking changes and 2.3.0 branch for breaking (non-backward compatible) changes.

Description of the PR

• Add thread-safe access to Alamofire Manager dictionary. Since we cannot assume the API client will always be invoked on the same thread.