etcdserver: add TestRecvSnapshot
*: add testutil pkg
etcdserver: not push ready to readyNode automatically
etcdserver: add TestRecvSlowSnapshot
Merge pull request #1118 from unihorn/133

etcdserver: add TestRecvSnapshot
raft: add Configure, AddNode, RemoveNode

Configure is used to propose config change. AddNode and RemoveNode is
used to apply cluster change to raft state machine. They are the
basics for dynamic configuration.
etcdserver: add AddNode, RemoveNode

AddNode and RemoveNode is used to propose config change to the cluster.
If succeeds, it will add/remove node from the cluster.
etcdserver: Config.Id -> Config.ID
raft: refine comment for raft.pendingConf
raft: use EntryType in protobuf
etcdserver: use ConfigType in protobuf