etcdserver: use Action to fix possible data race
*: init for on disk snap support
Merge pull request #1095 from coreos/nsnap

WIP *: init for on disk snap support
raft: introduce Node interface
Merge pull request #1093 from jonboulle/node_interface

Create Node Interface
Merge pull request #1097 from unihorn/130

etcdserver: use Action to fix possible data race
etcdserver: add nodeStepRecorder