Protostream must be able to compile the schemas incrementally

Methods from SerializationContext currently expect a self-contained FileDescriptorSource that has all the needed depencies so it can be parsed and resolved in one go. This limitation should be lifted so users can compile incrementally. Files that cannot be compiled due to missing dependencies should be ignored in current step until future invocations. A callback mechanism could be used to deliver the actual status of the compilation to the caller instead of throwing exceptions.