Internal RPC error with basic fdbcli connection

```
$ /usr/local/bin/fdbcli -C ./client.cluster
Using cluster file `./client.cluster'.
Internal Error @ fdbrpc/FlowTransport.actor.cpp 657:
atos -o fdbcli.debug -arch x86_64 -l 0x101fe7000 0x10233a5cd 0x1022a5e91 0x1022a514d 0x101feca78 0x102361916 0x102349631 0x1020bae5b 0x10200a6de 0x7fff59e48115
[lots more errors]
```

Server: Custom Docker container based on `ubuntu:16.04` that has installed `foundationdb-clients_5.1.5-1_amd64.deb` and `foundationdb-server_5.1.5-1_amd64.deb`. Using default config.

Client: Installed with macOS installer, `FoundationDB-5.1.5.pkg`. macOS 10.13.3.

Full `fdbcli` output and logs from server [here](https://gist.github.com/atombender/4dc4ff5948c4a28faa8d54cc5f122091).

Start with the failing fdbcli invocation and the server logs linked in the issue, then inspect fdbrpc/FlowTransport.actor.cpp around line 657. Compare the macOS client and Ubuntu 16.04 server setup described here. Done means a basic fdbcli connection no longer produces the internal RPC error.

Written by the indexing model from the issue text.