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.

test/shell/etcd-two-hosts-reversed.sh

@@ -0,0 +1,24 @@
+#!/bin/bash -e
+
+. "$(dirname "$0")/../util.sh"
+
+# run empty graphs, we're just testing etcd clustering
+$TIMEOUT "$MGMT" run --hostname h1 --tmp-prefix empty &
+pid1=$!
+sleep 15s	# let it startup
+
+$TIMEOUT "$MGMT" run --hostname h2 --seeds http://127.0.0.1:2379 --client-urls http://127.0.0.1:2381 --server-urls http://127.0.0.1:2382 --tmp-prefix empty &
+pid2=$!
+sleep 15s
+
+$(sleep 15s && kill -SIGINT $pid1)&	# send ^C to exit 1st mgmt! (reverse!)
+wait $pid1
+e=$?
+if [ $e -ne 0 ]; then
+	exit $e
+fi
+
+$(sleep 15s && kill -SIGINT $pid2)&	# send ^C to exit 2nd mgmt (reverse!)
+wait $pid2	# get exit status
+# if pid2 exits because of a timeout, then it blocked, and this is a bug!
+exit $?