Users might want to update the peerurl of the etcd member in several cases.
For example, if the IP address of the physical machine etcd running on is
changed, user need to update the adversite-pee-rurl accordingly.
This commit makes etcd support updating the advertise-peer-url of its members.
It is the correct thing to do to ensure that the communication is full
of out-of-date messages.
It results in that integration testing is very easy to throw MsgProp away,
and makes client wait until 5 min timeout. Sync interval and heartbeat are
increased to alleviate the traffic.
This creates a simple ID type (wrapped around uint64) to provide for
standard serialization/deserialization to a string (i.e. base 16
encoded). This replaces strutil so now that package is removed.
