Always close response channel when Watch is done.
Always close response channel when Watch is done.

Added a defer statement when the receiver channel passed to Watch is
set to close the channel. This permits the goroutine consuming that
channel to gracefully terminate when the channel will no longer return
any responses.

wangyumi/go-etcd,pengzj/go-etcd,pquerna/go-etcd,shafreeck/go-etcd,giantswarm/go-etcd,nordligulv/go-etcd,coreos/go-etcd,mbrukman/go-etcd,kmchen/go-etcd,stormltf/go-etcd,barakmich/go-etcd,zenoss/go-etcd,mischief/go-etcd,Yhgenomics/go-etcd