*: 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.
Merge pull request #1586 from xiangli-cmu/fix_node

*: add Advance interface to raft.Node