etcd: Rewrite embed etcd implementation

This is a giant cleanup of the etcd code. The earlier version was
written when I was less experienced with golang.

This is still not perfect, and does contain some races, but at least
it's a decent base to start from. The automatic elastic clustering
should be considered an experimental feature. If you need a more
battle-tested cluster, then you should manage etcd manually and point
mgmt at your existing cluster.

examples/lang/etcd-config0.mcl

@@ -0,0 +1,4 @@
+# sets a cluster parameter, safe to be called identically from multiple machines
+config:etcd "whatever" {
+	idealclustersize => 7,
+}