[Swift4] Use thread safe manager store dictionary

Created by: marcotta

PR checklist

• [ x] Read the contribution guidelines.
• [x ] 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\.
• Filed the PR against the correct branch: master, 3.3.x, 4.0.x. Default: master.
• [x ] Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

Use thread safe manager store dictionary Fix crash due to concurrent access of managerStore dictionary. Swift2 and Swift3 had been enhanced respectively with the following pull requests: #3873 #5610