v0.5.0-alpha.5

- extract storage interface in raft package
- print out more logs about raft and etcdserver behaviors
- support purging old WAL and snapshot files automatically
- optimize state machine processing in good path
- fix restore snapshot bug in raft
- fix the bug that may read empty state from WAL
- document how to change peer urls
- document the way to migrate member
- add external etcd-lock(implemented by ravigadde) link to the tool doc

Thank you to: Barak Michener, Ben Darnell, Brandon Philips, Brian Waldon, Johan
Euphrosine, ravigadde, Rob Szumski, Veres Lajos, Xiang Li, Yicheng Qin