This is an implementation of gRFC L112, currently in review at grpc/proposal#406. I had to rewrite a lot of the server-side request handling code to accommodate the new API, and I think the result is an improvement.

I changed a couple of existing tests because the previous behavior was incorrect:

• A test previously asserted that when a server sends an error, channelz counts it as a failed stream in the socket data. However, a stream should be counted as successful if the server explicitly ended it with trailers.
• Previously, the server sent an OUT_OF_RANGE status when receiving an invalid grpc-timeout value, but the library is not allowed to generate that status, so I changed it to INTERNAL.

The new test file shows a few over-simplified examples of using the server interceptor API.

This fixes #419.