Slow login with large number of nodes.

[russjones]

tsh ssh (and other commands like tsh ls) take a long time on large clusters.

For example, on a 4k node cluster with a local etcd instance as the backend and auth, proxy, node all running in the same process, tsh ssh can take over 6 seconds.

For a real world deployment where auth, proxy, node, and etcd all run on different machines, login can take over 12 seconds.