MINOR: h2: add the function to create a new stream

MINOR: h2: add the function to create a new stream

This one will be used by the HEADERS frame handler and maybe later by
the PUSH frame handler. It creates a conn_stream in the mux's connection.

The create streams are inserted in the h2c's tree sorted by IDs. The
caller is expected to have verified that the stream doesn't exist yet.