etcdserver: not add sender when it has existed
Merge pull request #1660 from yichengq/209

etcdserver: not add sender when it has existed
etcdserver: fix data race in cluster

The data race happens when etcd updates member attributes and fetches
member info in http handler at the same time.
doc: add backup/restore guide
Merge pull request #1662 from yichengq/211

etcdserver: fix data race in cluster
docs: no need to stop etcd while doing backup
docs: reword failure descriptions
docs: clarify rewriting
Merge pull request #1658 from jonboulle/doc_etcdctl_backup

Add doc for backup and force-new-cluster
version: bump to alpha.2
etcdserver: not get cluster info from self peer urls

Self peer urls have not started to serve at the time that it tries to
get cluster info, so it is useless to get cluster info from self peer
urls.
Merge pull request #1667 from yichengq/213

etcdserver: not get cluster info from self peer urls
integration: wait cluster to be stable before return launch

The wait ensures that cluster goes into the stable stage, which means that
leader has been elected and starts to heartbeat to followers.

This makes future client requests always handled in time, and there is no
need to retry sending requests.
Merge pull request #1668 from yichengq/214

integration: wait cluster to be stable before return launch
etcdserver: collapse shared readWAL logic