etcdserver: fix sync tests

This is to fix possible testing failures caused by sync tests.

Changes:
1. Get rid of time sleep operations, which introduces uncertainty.
2. Use fake Store.
Merge pull request #1098 from unihorn/131

fix sync tests
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