MultiplexV2 is a new connection multiplex header that supports multiplex both
RPC and streaming requests over the same Yamux connection.

MultiplexV2 was added in 0.8.0 as part of
#3892 .  So Nomad 0.11 can expect it to
be supported.  Though, some more rigorous testing is required before merging
this.

I want to call out some implementation details:

First, the current connection pool reuses the Yamux stream for multiple RPC calls,
and doesn't close them until an error is encountered.  This commit doesn't
change it, and sets the `RpcNomad` byte only at stream creation.

Second, the StreamingRPC session gets closed by callers and cannot be reused.
Every StreamingRPC opens a new Yamux session.

Co-Authored-By: Michael Schurter <mschurter@hashicorp.com>