Replace MuxConn with Yamux

This replaces the `packer/rpc/muxconn.go` (TCP multiplexer) with the much more performant and battle tested Yamux (github.com/hashicorp/yamux), used in Consul. The tests alone are 8x faster for `packer/rpc`:

```
packer f-yamux → go test ./packer/rpc/
ok      github.com/mitchellh/packer/packer/rpc    0.140s
packer f-yamux → gco master
Switched to branch 'master'
Your branch is up-to-date with 'origin/master'.
packer master → go test ./packer/rpc/
ok      github.com/mitchellh/packer/packer/rpc    0.930s
```

In addition to that, we get real backpressure now, so that should help with all the bugs (that are gone now) with out of memory issues.