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.
The usage of removed:
1. tell removed node about its removal explicitly using msgDenied
2. prevent removed node disrupt cluster progress by launching leader election
It is set when apply node removal, or receive msgDenied.
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.
