Add SynchronizedResolver property to Resolver protocol

[itsthejb]

• Alternate resolution to Assembly/Assembler need thread safety #221 / Add synchronizedResolver property to Assembler #445
• Expose a SynchronizedResolver property in the Resolver protocol.
• We use the Viper pattern, and have the architecture that a Resolver might be passed into some components (e.g. the Module), so that other components can be dynamically generated:

    public func assemble(container: Container) {
        container.register(FooModule.self) { resolver in
            FooModuleImplementation(resolver: resolver.synchronizedResolver)
        }
    }

• This causes thread-safety problems, however
• This PR is a possible solution, although it is a little untidy:
  • Even better would be somehow adding configurability/refactoring, so that the SynchronizedResolver also resolves dependencies using itself
  • Currently a SynchronizedResolver only works for the initial resolve(...) call

Happy to discuss other options, but we will be using this as a fix for the time being

[talron10]

Did it work for you? It seems like this will create a few synchronized resolvers while there should be only one, we are considering using this solution too

[itsthejb]

@talron10 Yes, it works - not crashing. But it's far from ideal, more like a workaround for now. If you have serious stability issues, it might be worth doing in the short-term, though