[Swift4] Use thread safe manager dictionary

[marcotta]

Description

Client crashes due to concurrent access of managerStore dictionary.
Swift2 and Swift3 had been enhanced respectively with the following pull requests:
#3873 #5610

Swagger-codegen version

2.3.1

Command line used for generation

swagger-codegen generate -i http://petstore.swagger.io/v2/swagger.json -l swift4

Related issues/PRs

#3873 #5610