This commit maintains a Set of all active sessions. This allows
tryShutdown() to gracefully stop the server properly (as
recommended in the Node HTTP2 documentation). The same Set of
sessions also allows forceShutdown() to be implemented.
This commit implements the following portion of the spec:
If Content-Type does not begin with "application/grpc",
gRPC servers SHOULD respond with HTTP status of
415 (Unsupported Media Type). This will prevent other
HTTP/2 clients from interpreting a gRPC error response,
which uses status 200 (OK), as successful.
Bjorn Stromberg
cjihrig