Use thread safe manager 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/swift4-petstore.sh and ./bin/security/swift-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\.
• [ x] 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. @wing328

Description of the PR

Fix crash due to concurrent access of managerStore dictionary. Swift2 and Swift3 had been enhanced respectively with the following pull requests: #3873 #5610 originally introduced by @jgavris