This patch introduces an additional ALPN protocol, grpc-exp, intended to
take preference to h2 and indicate to the server that the connection
contains only gRPC traffic. This allows servers and intermediate boxes
to distinguish gRPC from other HTTP/2 traffic.

The choice of grpc-exp as a protocol identifier indicates that this
scheme is currently experimental and should not be relied upon. The
protocol is not in the IANA TLS registry.

This patch also introduces client/server handshake tests that validate
the preferential treatment of grpc-exp in an end-to-end manner.