Introduce explicit behaviour for attempting to mutate ITransportConfiguration via the descriptor after it's first use

[stevejgordon]

Today, we generally treat the ITransportConfiguration as immutable once passed into the DistributedTransport constructor. The behaviour of modifying settings after this occurs is undefined. We propose to make the immutability explicit by introducing internal methods to mark the configuration as "locked" after it has been received by the ITransport. After this point, any calls to the descriptor methods will throw an exception, signalling the invalid operation.