*: add a Advance interface to raft.Node

Node set the applied to committed right after it sends out Ready to application. This is not
correct since the application has not actually applied the entries at that point. We add a
Advance interface to Node. Application needs to call Advance to tell raft Node its progress.
Also this change can avoid unnecessary copying when application is still applying entires but
there are more entries to be applied.
Advance() returns an error; we should include correct error handling in the example (at least an 'if err != nil { panic() }')