This commit adds an authentication mechanism to inter peer connection
(rafthttp). If the cert based peer auth is enabled and a new option
`--peer-cert-allowed-cn` is passed, an etcd process denies a peer
connection whose CN doesn't match.
pkg/ is a collection of utility packages used by etcd without being specific to etcd itself. A package belongs here
only if it could possibly be moved out into its own repository in the future.