Add support for ClientBase<TChannel>.CacheSetting

[mconnew]

This will help with #4186. We should do some follow-up work to have dotnet-svcutil emit the code to turn on caching as the only constructor on ClientBase<TChannel> that has caching turned on by default is the configuration based constructors which we don't support.

[mconnew]

I've added some additional caching so that there's a benefit even when you don't use the ClientBase<TChannel>.CacheSettings property. I added caching with a weak reference in TypeLoader so that repeated calls to ContractDescription.GetContract(Type contractType) will return the same instance. This means repeatedly instantiating a ChannelFactory for the same interface contract will return the same ContractDescription instance. There is a potential issue where if someone modifies the cached ContractDescription, they get the same modified instance returned instead of a fresh instance. To mitigate this, I made it so that only ChannelFactory uses the cached mechanism. ContractDescription.GetContract will continue to return a new instance each time. If someone is going to modify the ContractDescription, this is the way I would expect them to do so.