wal: sync before returning from create
raft: do not load empty state and ents
raft: add paper tests for section 5.4.1
etcdserver: try to listen on ports before initializing etcd server
Merge pull request #1587 from xiangli-cmu/fix_wal

wal: sync before returning from create
etcdhttp: use EcodePrevValueRequired when appropriate
integration: add keys API integration tests
Merge pull request #1592 from jonboulle/integration_tests

integration: add keys API integration tests
etcdserver: print out initial cluster members

It is moved from etcdmain pkg because the line should only be printed out
when etcd bootstraps at the first time.
Merge pull request #1593 from unihorn/200

etcdserver: print out initial cluster members
Merge pull request #1572 from xiangli-cmu/raft_test

raft: add paper tests for section 5.4.1
etcdhttp/etcdserver: support HEAD on /v2/keys/ namespace
etcdhttp: add HEAD support tests