Motivation:

The interceptors API has a context which, at the moment, only includes
the name of the RPC. This information is generally useful so should be
propagated to the server handler too. Information on the context can
also be extended later to include things like the identity of the remote
peer, or info about the state of the RPC.

Modifications:

- Rename 'ServerInterceptorContext' to 'ServerContext'
- Make the transport the source of the context and have that provide it
  to the 'listen' method. Propagate this through the server stack to the
  generated stubs.
- Update code generator to include the context
- Update generated code
- Update services

Results:

RPC handlers have a context provided by a transport