Split the large client/server.dart files into smaller pieces. This is in
preparation for splitting the HTTP/2 dependencies into a separate file
and make it easier to implement other transports.
Provide a hook for metadata providers that need to generate their
metadata for each RPC. An example is authorization, where the provider
may need to refresh a token before it can provide the header.
Add stackdriver logging examplei to demonstrate calling a Google API.
Updated other examples to protobuf 0.6.0 (protoc plugin 0.7.8).
Updated SDK requirement to Dart 1.24.3, which adds support for creating a SecurityContext that trusts built-in roots, and support for ALPN on macOS.
Implements the basic server interoperability tests for gRPC compliance.
Compressed tests, TLS, and authentication are not implemented yet, but
basic test cases pass against the C++ gRPC interoperability client.