Prepare EtcdHelper to extract more data from Node

In order to support graceful deletion, the resource object will
need access to the TTL value in etcd.  Also, in the future we
may want to get the creation index (distinct from modifiedindex)
and expose it to clients.  Change EtcdResourceVersioner to be
more type specific (objects vs lists) and provide a default
implementation that relies on the internal API convention.

Also, rename etcd_tools.go to etcd_helper.go and split a few
things up.

pkg/registry/event/registry_test.go

@@ -37,7 +37,7 @@ var testTTL uint64 = 60
 func NewTestEventEtcdRegistry(t *testing.T) (*tools.FakeEtcdClient, generic.Registry) {
 	f := tools.NewFakeEtcdClient(t)
 	f.TestIndex = true
-	h := tools.EtcdHelper{f, testapi.Codec(), tools.RuntimeVersionAdapter{testapi.MetadataAccessor()}}
+	h := tools.NewEtcdHelper(f, testapi.Codec())
 	return f, NewEtcdRegistry(h, testTTL)
 }