*: support updating advertise-peer-url
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.
raft: update unstable when calling stableTo with 0

It should update unstable in this case because it may happen that raft
only writes entry 0 into stable storage.
raft: nodes return sorted ids

This makes raft.softState return the same result when its soft state is
not changed.
Merge pull request #1692 from yichengq/221

raft: nodes return sorted ids
Merge pull request #1679 from xiang90/peerurl

update peer url
Merge pull request #1689 from yichengq/219

raft: update unstable when calling stableTo with 0
etcdserver: use member instead of node at etcd level
Merge pull request #1694 from xiang90/use_member

etcdserver: use member instead of node at etcd level
Add fast leadership handover
Allow fast-switch by name instead of id
Add `--heap-debug` flag
Allow setting `--snap-count` to 0 to disable snapshots
Notify client when not talking to leader
Print out each request time spent at server
Allow some buffering / queueing on sending
Add blade gc support
Allow more buffering on sending.