integration: wait for roundTrip dial timeout

master breaks from time to time because there is left goroutine:
```
--- FAIL: TestGoroutinesRunning (0.01 seconds)
z_last_test.go:54: num goroutines = 1
z_last_test.go:56: Too many goroutines.
z_last_test.go:58: 1 instances of:
net/http.(*persistConn).roundTrip(0xc2081ba000, 0xc20814a950, 0x0, 0x0, 0x0)
/home/travis/.gvm/gos/go1.3.3/src/pkg/net/http/transport.go:1015 +0x970
net/http.(*Transport).RoundTrip(0xc208048680, 0xc20810a1a0, 0x430272, 0x0, 0x0)
/home/travis/.gvm/gos/go1.3.3/src/pkg/net/http/transport.go:208 +0x60e
net/http.send(0xc20810a1a0, 0x7fda945076d8, 0xc208048680, 0x1b, 0x0, 0x0)
/home/travis/.gvm/gos/go1.3.3/src/pkg/net/http/client.go:195 +0x62e
net/http.(*Client).send(0xc208086ba0, 0xc20810a1a0, 0x1b, 0x0, 0x0)
/home/travis/.gvm/gos/go1.3.3/src/pkg/net/http/client.go:118 +0x201
net/http.(*Client).doFollowingRedirects(0xc208086ba0, 0xc20810a1a0, 0x9e0520, 0x0, 0x0, 0x0)
/home/travis/.gvm/gos/go1.3.3/src/pkg/net/http/client.go:343 +0xd32
net/http.(*Client).Do(0xc208086ba0, 0xc20810a1a0, 0x11, 0x0, 0x0)
/home/travis/.gvm/gos/go1.3.3/src/pkg/net/http/client.go:153 +0x1e2
github.com/coreos/etcd/etcdserver.httpPost(0xc208086ba0, 0xc208167bc0, 0x1b, 0xe0f9af516b29463f, 0xc2084ca420, 0x29, 0x29, 0xba3138)
/home/travis/gopath/src/github.com/coreos/etcd/gopath/src/github.com/coreos/etcd/etcdserver/cluster_store.go:197 +0x267
github.com/coreos/etcd/etcdserver.send(0xc208086ba0, 0x7fda94508e38, 0xc2080437e0, 0x3, 0xf1fc26c5ecfe5cc6, 0x993a8313fd55ce3, 0x1, 0x0, 0x0, 0x0, ...)
/home/travis/gopath/src/github.com/coreos/etcd/gopath/src/github.com/coreos/etcd/etcdserver/cluster_store.go:176 +0x558
created by github.com/coreos/etcd/etcdserver.func·001
/home/travis/gopath/src/github.com/coreos/etcd/gopath/src/github.com/coreos/etcd/etcdserver/cluster_store.go:140 +0x1c5
```
It will be fixed entirely if Sender has Stop function to recycle all goroutines.
The first commit in this issue fails with https://travis-ci.org/coreos/etcd/builds/38673516