Fix Race Condition between Configuration and Adapt()

[DocSvartz]

Based on these tests and #86, #138

Depending on the speed and order of execution (configuration and Adapt()), one of the following situations can occur:

1. Incomplete configuration (the main cause of errors in the tests mentioned at the beginning);
2. "Incomplete cleanup" - Exception: "TypeAdapter.Adapt was already called, please clone or create a new TypeAdapterConfig."
3. Exception to collection modification during enumeration.

Additional problem:
The current configuration design does not provide information about configuration completion.

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1. Thread safety creating a new configuration:

 TypeAdapterConfigConcurrency<WhenAddingCustomMappings.SimplePoco, WeirdPoco>
                .NewConfig(cfg =>
                {
                    cfg
                        .Map(dest => dest.IHaveADifferentId, src => src.Id)
                        .Map(dest => dest.IHaveADifferentId, src => src.Id)
                        .Map(dest => dest.MyNamePropertyIsDifferent, src => src.Name)
                        .Ignore(dest => dest.Children);
                });

2. Thread safety loading configurations:

TypeAdapterConfig.GlobalSettings.ScanConcurrency(Assembly.GetExecutingAssembly());

[DocSvartz]

@andrerav @stagep See if this is necessary at all?
Actually, I don't fully understand the meaning of the initial tests :)