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-clustersize.sh

@@ -0,0 +1,40 @@
+#!/bin/bash
+
+. "$(dirname "$0")/../util.sh"
+
+set -o errexit
+set -o pipefail
+
+if ! command -v etcdctl >/dev/null; then
+	echo "Missing etcdctl, skipping"
+	exit 0
+fi
+
+#mkdir /tmp/mgmt/{A..E}
+
+# kill servers on error/exit
+trap 'pkill -9 mgmt' EXIT
+
+$TIMEOUT "$MGMT" run --hostname h1 --tmp-prefix --no-pgp empty &
+$TIMEOUT "$MGMT" run --hostname h2 --tmp-prefix --no-pgp --seeds http://127.0.0.1:2379 --client-urls http://127.0.0.1:2381 --server-urls http://127.0.0.1:2382 empty &
+$TIMEOUT "$MGMT" run --hostname h3 --tmp-prefix --no-pgp --seeds http://127.0.0.1:2379 --client-urls http://127.0.0.1:2383 --server-urls http://127.0.0.1:2384 empty &
+
+# wait for everything to converge
+sleep 30s
+
+ETCDCTL_API=3 etcdctl --endpoints 127.0.0.1:2379 put /_mgmt/chooser/dynamicsize/idealclustersize 3
+
+$TIMEOUT "$MGMT" run --hostname h4 --tmp-prefix --no-pgp --seeds http://127.0.0.1:2379 --client-urls http://127.0.0.1:2385 --server-urls http://127.0.0.1:2386 empty &
+$TIMEOUT "$MGMT" run --hostname h5 --tmp-prefix --no-pgp --seeds http://127.0.0.1:2379 --client-urls http://127.0.0.1:2387 --server-urls http://127.0.0.1:2388 empty &
+
+# wait for everything to converge
+sleep 30s
+
+test "$(ETCDCTL_API=3 etcdctl --endpoints 127.0.0.1:2379 member list | wc -l)" -eq 3
+
+ETCDCTL_API=3 etcdctl --endpoints 127.0.0.1:2379 put /_mgmt/chooser/dynamicsize/idealclustersize 5
+
+# wait for everything to converge
+sleep 30s
+
+test "$(ETCDCTL_API=3 etcdctl --endpoints 127.0.0.1:2381 member list | wc -l)" -eq 5