Upgrade to Multiplex v2 protocol

[schmichael]

#3892 introduced a new RpcMultiplexV2 stream multiplexer to Nomad's RPC protocol. Since all clients and servers in a cluster must support v2 before it can be used, we could not immediately switch. The benefit of v2 is that you don’t have to establish a new TCP connection for streaming RPCs.

We would have to update the conn pool to use the V2 type: https://github.com/hashicorp/nomad/blob/v0.10.2/helper/pool/pool.go#L333 and then change the streaming RPC implementation to use the pool: https://github.com/hashicorp/nomad/blob/v0.10.2/nomad/rpc.go#L531-L606. The client also has similar logic: https://github.com/hashicorp/nomad/blob/v0.10.2/client/rpc.go#L150. There is probably some DRYing to do.

Testing

Test a mix of 0.X and 0.X-1 servers and clients where X is the first version of Nomad to use RpcMultiplexV2 by default. Initiate the streaming connection as a matrix of client/server and old/new version.

[github-actions]

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