listen on multiple sockets

etcd should be able to listen on a list of ip:port combinations. Then we can e.g. seamlessly listen on the old and new IANA-assigned ports

--addr should take a comma separated list, e.g.:
```
unix:///var/run/etcd.sock,0.0.0.0:4001,127.0.0.1:5002,10.0.0.1:8080
```
yes, rolling forward.